ramips: add support for ASUS RT-N56U B1

ramips: add support for ASUS RT-N56U B1

Specifications:

SoC: MediaTek MT7621ST (880 MHz)
FLASH: 16 MiB (Macronix MX25L12835FM2I-10G)
RAM: 128 MiB (Nanya NT5CB64M16FP-DH)
WiFi: MediaTek MT7603EN bgn 2x2:2
WiFi: MediaTek MT7612EN an 2x2:2
BTN: Reset, WPS
LED: - Power
- WiFi 2.4 GHz
- WiFi 5 GHz
- WAN
- LAN {1-4}
- USB {1-2}
UART: UART is present as pin hole next to the aluminium capacitor.
3V3 - RX - GND - TX / 115200-8N1
3V3 is the nearest on the aluminium capacitor and nut hole (pin1).
USB: 2 ports
POWER: 12VDC, 1.5A (Barrel 5.5x2.1)

Installation:

Via TFTP:
    Set your computers IP-Address to 192.168.1.75
    Power up the Router with the Reset button pressed.
    Release the Reset button after 5 seconds.
    Upload OpenWRT sysupgrade image via TFTP:
    tftp -4 -v -m binary 192.168.1.1 -c put IMAGE

MAC addresses:

0x4     *:98  2g/wan, label
0x22    *:9c
0x28    *:98
0x8004  *:9c  5g/lan

Though addresses are written to 0x22 and 0x28, it appears that the
vendor firmware actually only uses 0x4 and 0x8004. Thus, we do the
same here.

Signed-off-by: Pavel Chervontsev <cherpash@gmail.com>
[add MAC address overview, add label-mac-device, fix IMAGE_SIZE]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>