bmips: bcm6328: add support for D-Link DSL-2750B rev B1

bmips: bcm6328: add support for D-Link DSL-2750B rev B1

The D-Link DSL-2750B rev B1 (AW4339U) is a wifi fast ethernet router, 2.4 GHz single band
with two external antennas.

This ports the device from old target bcm63xx/generic to bmips/bcm6328.
The hardware is the same of D-Link DSL-2740B rev F1 and DSL-2741B rev F1, plus a usb2 port.

Hardware:
 - SoC: Broadcom BCM63281
 - CPU: single core BMIPS4350 @ 320Mhz
 - RAM: 64 MB (Nanya NT5TU32M16DG)
 - Flash: 8 MB NOR (Macronix MX25L6406ENI-12G)
 - Ethernet LAN: 4x 100Mbit (Broadcom BCM63281)
 - Wifi 2.4 GHz: 802.11bgn (Atheros AR9287)
 - USB: 1x 2.0
 - Buttons: 3x
 - LEDs: 10x
 - 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: Samuele Longhi <agave@dracaena.it>