brcm63xx: add support for ADB P.DG A4001N A-000-1A1-AX

ADB P.DG A4001N A-000-1A1-AX a.k.a. Telecom Italia ADSL2+ Wi-Fi N (AGPWI)
has the same PCB as the OpenWrt's ADB P.DG A4001N1 with LEDs connected
to different GPIO PINs in active low configuration.

OpenWrt's ADB P.DG A4001N image is made for the ADB P.DG A4001N A-000-1A1-AE.
It has different LEDs configuration and flash size/layout
w.r.t the ADB P.DG A4001N A-000-1A1-AX.

Hardware:
* Board ID: 96328avng
* SoC: Broadcom BCM6328
* RAM DDR2-800: 32 Mbyte - winbond W9725G6KB-25
* Serial flash: 16 Mbyte - MXIC MX25L 12845EMI-10G
* Ethernet: 4x Ethernet 10/100 baseT
* Wifi 2.4GHz: Broadcom Corporation BCM43224/5 Wireless Network Adapter (rev 01)
* LEDs: 2x Power, 2x ADSL, 2x Internet, 2x Wi-Fi, 2x Service
* Buttons: 1x Reset, 1x WPS (named WiFi/LED)
* UART: 1x TTL 115200n8, TX  NC  RX, on J5 connector (short R192 and R193)
                         NC  GND NC

Installation via CFE:
* Stock CFE has to be overwriten with one for 96328avng boards that can upload
  .bin images with no signature check (cfe-A4001N-V0000_96328avng.bin)
* connect a serial port to the board
* Stop the boot process after power on by pressing enter
* set static IP 192.168.1.2 and subnet mask 255.255.255.0
* navigate to http://192.168.1.1/


* upload the OpenWrt image file

Signed-off-by: Daniele Castro <danielecastro@hotmail.it>
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>