ramips: add support for TP-Link RE650 v2

ramips: add support for TP-Link RE650 v2

TP-Link RE650 v2 is largely similar to v1 that
is already supported by OpenWrt. Notable differences
is differnt SPI Flash - 8 MB instead of 16 MB
(from cFeon instead of Winbond) and a different
configuration of PCIE connections to wifi chips.
Otherwise it's largely the same product as v1

Hardware specification:

- SoC 880 MHz - MediaTek MT7621AT
- 128 MB of DDR3 RAM
- 8 MB - cFeon QH64A-104HIP
- 4T4R 2.4 GHz - MediaTek MT7615E
- 4T4R 5 GHz - MediaTek MT7615E
- 1x 1 Gbps Ethernet - MT7621AT integrated
- 7x LEDs (Power, 2G, 5G, WPS(x2), Lan(x2))
- 4x buttons (Reset, Power, WPS, LED)
- UART pinout - GND, RX, TX, labeled in the middle of the PCB,
  requires soldering because they're not through holes.

  Serial console @ 57600,8n1

Flash instructions:

Upload
openwrt-ramips-mt7621-tplink_re650-v2-squashfs-factory.bin
from the RE650 web interface.

TFTP recovery to stock firmware:
I didn't try recovering back to the stock firmware, however,
if there is such process for other RExxx devices, it seems like
it could be similar here.

Signed-off-by: Marcin Gordziejewski <openwrt@flicksfix.com>