bmips: add support for Comtrend WAP-5813n

bmips: add support for Comtrend WAP-5813n

The Comtrend WAP-5813n is a wifi gigabit router, 2.4 GHz single band with
two external antennas.

Hardware:
 - SoC: Broadcom BCM6369
 - CPU: dual core BMIPS4350 @ 400Mhz
 - RAM: 64 MB DDR
 - Flash: 8 MB parallel NOR
 - LAN switch: Broadcom BCM53115, 5x 1Gbit
 - Wifi 2.4 GHz: miniPCI Broadcom BCM4322 802.11bgn
 - USB: 1x 2.0 (optional)
 - Buttons: 3x (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>
(cherry picked from commit c3b1ef2dfd)