ipq806x: add support for Linksys EA7500 V1

ipq806x: add support for Linksys EA7500 V1

This patch adds support for the Linksys EA7500 V1 router.

Specification:
 - CPU: Qualcomm IPQ8064
 - RAM: 256MB
 - Flash: NAND 128MB
 - WiFi: QCA9982 an+ac + QCA9983 bgn
 - Ethernet: 5 GBE Ports (WAN+ 4xLAN) (QCA8337)
 - USB: 1x USB 3.0 1x USB2.0
 - Serial console: RJ-45 115200 8n1 (1V8 Voltage level)
 - 2 Buttons
 - 1 LED

Known issues:
 - Some devices won't flash via web gui

Installation:
- Newer stock images doesn't allow to install custom firmware.
- Please downgrade software to 1.1.2 version. Official firmware:
https://downloads.linksys.com/downloads/firmware/FW_EA7500_1.1.2.172843_prod.gpg.img
- Do it two times to downgrade all stored images.
- Apply factory image via web-gui.

Serial + TFTP method:
 - downgrade to 1.1.2 two times
 - connect ehternet and serial cable
 - set ip address of tftp server to 192.168.1.254
 - put openwrt factory image to tftp folder and rename it to macan.bin
 - stop device while booting in u-boot
 - run command: "run flashimg"
 - run command: "setenv boot_part 1"
 - run command "saveenv"
 - reset

Back to stock:
- Please use old non-gpg image like this 1.1.2:
https://downloads.linksys.com/downloads/firmware/FW_EA7500_1.1.2.172843_prod.img
- ssh to router and copy image to tmp
- use sysupgrade -n -F

Tested by github users: @jack338c and @grzesiczek1

Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
[removed i2c4_pins, mdio0_pins, nand_pins, rgmii2_pins from DTSI]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>