ramips: add support for EnGenius ESR600

ramips: add support for EnGenius ESR600

The EnGenius ESR600 is a dual band wireless router with a 4-port gigabit
Ethernet switch, a gigabit Ethernet WAN port and a USB port.

Specification:

- Bootloader:	U-Boot
- SoC:		MediaTek MT7620A (600 MHz)
- Flash:	16MB, Macronix MX25L12845E
- RAM:		64MB, Nanya NT5TU32M16DG-AC
- Serial:	115200 baud, no header, 3.3V
  		J2: Vcc (arrow), Gnd, Tx, Rx
- USB:		USB 2, 5V
- Ethernet:	5 x 1 Gb/s 4 LAN 1 WAN, Atheros AR8327
- WiFi0:	5 GHz 802.11 b/g/n Ralink RT5592N
  		300 Mb/s, 2T2R
- WiFi1:	2.4 GHz 802.11 b/g/n integrated
  		300 Mb/s, 2T2R
- Antennas:	2 per radio, internal
- LEDs:		1 programmable power (amber)
  		2 programable radio (blue)
		1 programable WPS-5G (blue)
		1 non-programable WAN activity (blue)
		1 unconfigured WPS-2.4G (amber)
- Buttons:	GPIO: Reset, WPS

Installation:

Use the OEM web interface to install the ...-factory.dlf image.
Use the OpenWRT ...-sysupgrade.bin image for future upgrades.

The J2 serial port can be accessed either by soldering in a header,
standard 0.1" spacing, or by using pogo-pins against the back side.

As configured by the OEM, the U-Boot boot delay is short, however quickly
typing "1" leads to the U-Boot "System load Linux to SDRAM via TFTP"
prompt.  The TFTP client is configured by default with
     client:   192.168.99.9
     server:   192.168.99.8
     filename: uImageESR600
It will load an OpenWRT initramfs kernel with this method.

Known issues:

1) Only the ports externally labeled WAN, LAN3 and LAN4 are operational.
LAN1 and LAN2 do not appear to power up. This issue is also present
in the Lava LR25G001.

2) The amber WPS-2.4G LED, in the same lightguide as the blue WPS-5G LED,
is not configured in the Device Tree specification.

3) The blue WAN activity LED is not configured in the Device Tree
specification as this causes the AR8327 switch to fail to initialize.

Signed-off-by: Nick Briggs <nicholas.h.briggs@gmail.com>
[merge conflict in 02_network]
Signed-off-by: Petr Štetiar <ynezz@true.cz>