ramips: add support for NETGEAR WAC124

ramips: add support for NETGEAR WAC124

The WAC124 hardware appears to be identical to R6260/R6350/R6850.

SoC:   MediaTek MT7621AT
RAM:   128M DDR3
FLASH: 128M NAND (Macronix MX30LF1G18AC)
WiFI:  MediaTek MT7603 bgn 2T2R
       MediaTek MT7615 nac 4T4R
ETH:   SoC Integrated Gigabit Switch (1x WAN, 4x LAN)
USB:   1x USB 2.0
BTN:   Reset, WPS
LED:   Power, Internet, WiFi, USB (all green)

Installation:
The factory image can be flashed from the stock firmware web interface
or using nmrpflash. With nmrpflash it is also possible to revert to
stock firmware.

Signed-off-by: Jan Hoffmann <jan@3e8.eu>