bmips: add support for Comtrend AR-5381u

bmips: add support for Comtrend AR-5381u

The Comtrend AR-5381u is a wifi fast ethernet router, 2.4 GHz single band with
two internal antennas.

Hardware:
 - SoC: Broadcom BCM6328
 - CPU: single core BMIPS4350 @ 320Mhz
 - RAM: 64 MB DDR
 - Flash: 16 MB SPI NOR
 - Ethernet LAN: 4x 100Mbit
 - Wifi 2.4 GHz: miniPCI Broadcom BCM43225 802.11bgn
 - USB: 1x 2.0
 - Buttons: 1x (reset)
 - LEDs: yes
 - UART: yes

Installation via CFE web UI:
  1. Power off the router.
  2. Press reset button near the power switch.
  3. Keep it pressed while powering up during ~20+ seconds.
  4. Browse to http://192.168.1.1 and upload the firmware.
  5. Wait a few minutes for it to finish.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>