bmips: add support for TP-Link TD-W8968 V3

bmips: add support for TP-Link TD-W8968 V3

TP-Link TD-W8968 v3 is an 300Mbps Wireless N USB ADSL2+ Modem Router based on
Broadcom BCM6318 SoC.

Hardware:
   CPU:          Broadcom BCM6318, 333 MHz, 1 core
   Flash:        8MB
   RAM:          64 MB
   Ethernet:     4x 10/100 Mbps
   Wireless:     802.11b/g/n, BCM43217
   LEDs/Buttons: 10x / 3x
   USB:          1x 2.0

Flash instructions:
* Assign static IP 192.168.1.100 to PC
* Unplug the power source
* Press the RESET button at the router, don't release it yet!
* Plug the power source. Wait for some seconds
* Release the RESET button
* Browse to http://192.168.1.1
* Upload the openwrt-bmips-bcm6318-tp-link_td-w8968-v3-squashfs-cfe.bin file
* Wait some minutes until the firmware upgrade finish.

Signed-off-by: Mathesh Velayudan <123v.mathesh@gmail.com>