ramips: mt7621: Add new device AsiaRF AP7621-NV1

ramips: mt7621: Add new device AsiaRF AP7621-NV1

SoC:    Mediatek MT7621A
CPU:    4x 880Mhz
Cache:  32 KB I-Cache and 32 KB D-Cach
        256 KB L2 Cache (shared by Dual-Core)
RAM:    DDR3 512MB 16bits BUS
FLASH:  16MB
Switch: Mediatek Gigabit Switch (2 x LAN, 1 x WAN)
POE:    (1x PD, 2x PSE)
USB:    1x 3.0
PCI:    3x Mini PCIe (3 USB2.0 + 2 x UIM interface)
GPS:    Quectel L70B
SIM:    2 Slots
BTN:    Reset
LED:    - Power
        - Ethernet
        - Wifi
        - USB
UART:  UART is present as Pads with throughholes on the PCB.
       They are located on left side.
       3.3V - RX - GND - TX / 57600-8N1
       3.3V is the square pad

Installation
------------
The stock image is a modified openwrt and can be overflashed via sysupgrade -F

Signed-off-by: Daniel Danzberger <daniel@dd-wrt.com>
[merge conflict in mt7621.mk]
Signed-off-by: Petr Štetiar <ynezz@true.cz>