ath79: add support for Fritz!Box 4020

ath79: add support for Fritz!Box 4020

This commit adds support for the AVM Fritz!Box 4020 WiFi-router.

SoC:   Qualcomm Atheros QCA9561 (Dragonfly) 750MHz
RAM:   Winbond W971GG6KB-25
FLASH: Macronix MX25L12835F
WiFi:  QCA9561 b/g/n 3x3 450Mbit/s
USB:   1x USB 2.0
IN:    WPS button, WiFi button
OUT:   Power LED green, Internet LED green, WLAN LED green,
       LAN LED green, INFO LED green, INFO LED red
UART:  Header Next to Black metal shield
       Pinout is 3.3V - RX - TX - GND (Square Pad is 3.3V)
       The Serial setting is 115200-8-N-1.

Tested and working:
 - Ethernet (LAN + WAN)
 - WiFi (correct MAC)
 - Installation via EVA bootloader
 - OpenWRT sysupgrade
 - Buttons
 - LEDs

The USB port doesn't work. Both Root Hubs are detected as having 0 Ports:

[    3.670807] kmodloader: loading kernel modules from /etc/modules-boot.d/*
[    3.723267] usbcore: registered new interface driver usbfs
[    3.729058] usbcore: registered new interface driver hub
[    3.734616] usbcore: registered new device driver usb
[    3.744181] ehci_hcd: USB 2.0 'Enhanced' Host Controller (EHCI) Driver
[    3.758357] SCSI subsystem initialized
[    3.766026] ehci-platform: EHCI generic platform driver
[    3.771548] ehci-platform ehci-platform.0: EHCI Host Controller
[    3.777708] ehci-platform ehci-platform.0: new USB bus registered, assigned bus number 1
[    3.788169] ehci-platform ehci-platform.0: irq 48, io mem 0x1b000000
[    3.816647] ehci-platform ehci-platform.0: USB 2.0 started, EHCI 0.00
[    3.824001] hub 1-0:1.0: USB hub found
[    3.828219] hub 1-0:1.0: config failed, hub doesn't have any ports! (err -19)
[    3.835825] ehci-platform ehci-platform.1: EHCI Host Controller
[    3.842009] ehci-platform ehci-platform.1: new USB bus registered, assigned bus number 2
[    3.852481] ehci-platform ehci-platform.1: irq 49, io mem 0x1b400000
[    3.886631] ehci-platform ehci-platform.1: USB 2.0 started, EHCI 0.00
[    3.894011] hub 2-0:1.0: USB hub found
[    3.898190] hub 2-0:1.0: config failed, hub doesn't have any ports! (err -19)
[    3.908928] usbcore: registered new interface driver usb-storage
[    3.915634] kmodloader: done loading kernel modules from /etc/modules-boot.d/*

A few words about the shift-register:

AVM used a trick to control the shift-register for the LEDs with only 2
pins, SERCLK and MOSI. Q7S, normally used for daisy-chaining multiple
shift-registers, pulls the latch, moving the shift register-state to
the storage register. It also pulls down MR (normally pulled up) to
clear the storage register, so the latch gets released and will not be
pulled by the remaining bits in the shift-register. Shift register is
all-zero after this.

For that we need to make sure output 7 is set to high on driver probe.
We accomplish this by using gpio-hogging.

Installation via EVA:
In the first seconds after Power is connected, the bootloader will
listen for FTP connections on 169.254.157.1 (Might also be 192.168.178.1).
Firmware can be uploaded like following:

  ftp> quote USER adam2
  ftp> quote PASS adam2
  ftp> binary
  ftp> debug
  ftp> passive
  ftp> quote MEDIA FLSH
  ftp> put openwrt-sysupgrade.bin mtd1

Note that this procedure might take up to two minutes. After transfer is
complete you need to powercycle the device to boot OpenWRT.

Signed-off-by: David Bauer <mail@david-bauer.net>