ramips: add support for Netgear WN3100RPv2

ramips: add support for Netgear WN3100RPv2

This patch adds support for the Netgear WN3100RPv2
http://www.netgear.com/support/product/wn3100rpv2.aspx

Specifications:
- SoC: MediaTek MT7620A (580MHz, ramips)
- RAM: 32MB DDR
- Storage: 8MB NOR SPI flash
- Wireless: builtin MT7620A, 2x2:2 with u.FL connectors
- Ethernet: 1x100M
- Stock firmware based on OpenWRT Kamikaze

Like the EX2700, the bootloader expects a secondary image signature,
see https://forum.openwrt.org/viewtopic.php?pid=312577#p312577

This device seems to be same hardware as a WN3000RPv3

Flash instructions:
- Use the Netgear WebUI to upgrade to OpenWRT using the factory image
  (see note below),
- Use the sysupgrade image for upgrading versions from OpenWRT,
- TFTP recovery procedure can be used to flash the factory image
  (preferred method).

Note:
- The WebUI may not reboot automatically, wait at least 5 minutes before
  powercycling the device

Flashing using TFTP:
- Set you IP address to 192.168.1.10/24 (no gateway)
- Connect your machine to the Ethernet port
- Power off the device and wait for 10 seconds,
- Hold the reset button and power on the device (do not release reset),
- Hold the reset button until the green light is flashing (Approx. 15s)
- launch tftp, set mode to binary and connect to 192.168.1.1
- put the factory firmware image
- All leds will switch off (like a power off), this is normal
- Wait for the device to reboot in the new OpenWRT image (max 5 mins)
- The first boot will take longer than usual.
- After boot, the Device IP on the ethernet port is 192.168.1.1

Signed-off-by: Rodolphe de Saint Léger <rdesaintleger@gmail.com>
[drop unneeded includes in dts, wrap commit message]
Signed-off-by: Sungbo Eo <mans0n@gorani.run>