ramips: add support for Netgear R6350

Netgear R6350 is a wireless router, aka Netgear AC1750.

Specification:
- SoC: Mediatek MT7621AT (2 CPU cores, 4 threads)
- RAM: 128MiB (Nanya NT5CC64M16GP-DI)
- ROM: 128MiB NAND Flash (Macronix MX30LF1G18AC-TI)
- Wireless:
   for 11b/g/n (upto 300Mbps): MT7603
   for 11a/ac  (upto 1450Mbps) : MT7615, is not avaliable now
- Ethernet LAN speed: up to 1000Mbps
- Ethernet LAN ports: 4
- Ethernet WAN speed: up to 1000Mbps
- Ethernet WAN ports: 1
- USB ports: 1 (USB 2.0)
- LEDs: 4 (all can be controlled by SoC's GPIO)
- buttons: 2
- serial ports: unknown

Installation through telnet:
  - Copy kernel.bin and rootfs.bin to a USB flash disk,
    plug to usb port on the router.
  - Enable telnet with link: http://192.168.1.1/setup.cgi?todo=debug


    (login if required, default: admin password)
  - You will see "Debug Enabled!"
  - Telnet 192.168.1.1 and login with "root"
  - ls /mnt/shares/ to find out path of your USB disk.
    'myUdisk' for example.
  - cd /mnt/shares/myUdisk
  - mtd_write write rootfs.bin Rootfs
  - mtd_write write kernel.bin Kernel
  - reboot

recovery when bricked:
  nmrpflash can be used to recover to the netgear firmware
  if a broken image was flashed.

The SC_PART_MAP partition suggests that an on flash partition table
exists. After implementing a partition parser/builder for the sercom
partition format, the definitions don't match the flash layout used by
the stock firmware.

It either means the partition format has not yet been completely
understood or it isn't used by the stock firmware. For now, use fixed
partitions instead.

Signed-off-by: NOGUCHI Hiroshi <drvlabo@gmail.com>
[apply latest ramips changes and document the on flash partition map
issues]
Signed-off-by: Mathias Kresin <dev@kresin.me>