bmips: add support for Actiontec T1200H

bmips: add support for Actiontec T1200H

Specifications:
- SoC: Broadcom BCM63168 dual 400MHz MIPS
- Flash: 128MB NAND ESMT F59L1G81A
- RAM: 128MB DDR3
- Ethernet: 5x 1000M
- Wifi: BCM435F
- 1x USB 2.0 port
- 2x Button
- 11x LED
- Coax HPNA (unsupported)
- RJ11 xDSL (unsupported)

Install instructions:
The booloader will only install signed firmware so the image has to
be manually flashed.

- Set up a TFTP server on 192.168.1.0/24 to serve the initramfs image
- Interrupt the bootloader and run from RAM with
  r 192.168.1.100:openwrt-bmips-bcm63268-actiontec_t1200h-initramfs.elf
- On the openwrt console, copy the wfi image using wget/tftp/scp, i.e
  scp user@192.168.1.100:~/openwrt/bin/targets/bmips/bcm63268/openwrt-bmips-bcm63268-actiontec_t1200h-squashfs-cfe.bin /tmp/
- Erase half the flash partition where openwrt will be installed with
  flash_erase -j /dev/mtd1 0 491
- Flash the openwrt image with
  nandwrite -p /dev/mtd1 /tmp/openwrt-bmips-bcm63268-actiontec_t1200h-squashfs-cfe.bin
- Reboot
- Interrupt the bootloader again and use the c command to boot from
  previous firmware if vendor image is loaded

Signed-off-by: Kyle Hendry <kylehendrydev@gmail.com>
[Minor improvements to DTS file]
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>