ramips: add support for Netgear R6120

This patch adds support for the Netgear R6120, aka Netgear AC1200.

Specification:
- SoC: MediaTek MT7628 (580 MHz)
- Flash: 16 MiB
- RAM: 64 MiB
- Wireless: 2.4Ghz(builtin) and 5Ghz (MT7612E)
- LAN speed: 10/100
- LAN ports: 4
- WAN speed: 10/100
- WAN ports: 1
- Serial baud rate of Bootloader and factory firmware: 57600

To flash use nmrpflash with the provided factory.img.
Flashing via webinterface will not work, for now.

Signed-off-by: Ludwig Thomeczek <ledesrc@wxorx.net>

target/linux/ramips/base-files/lib/upgrade/platform.sh

@@ -132,6 +132,7 @@ platform_check_image() {
 	psr-680w|\
 	px-4885-4M|\
 	px-4885-8M|\
+	netgear,r6120|\
 	rb750gr3|\
 	re6500|\
 	rp-n53|\