ramips: add support for Joowin WR758AC V1 and V2

ramips: add support for Joowin WR758AC V1 and V2

This commit adds support for Joowin (aka Comfast) WR758AC V1 and V2
devices.

Both have the same wall AP/repeater form factor and differ only
in the 5Ghz chipset (V1 has MT7662, V2 has MT7663).

OpenWrt developers forum page:
https://forum.openwrt.org/t/87355

Specifications:

 - CPU: MediaTek MT7628AN (580MHz)
 - Flash: 8MB
 - RAM: 64MB DDR2
 - 2.4 GHz: 802.11b/g/n (MT7603)
 - 5 GHz: 802.11ac (V1 has MT7662, V2 has MT7663)
 - Antennas: 4x external single band antennas
 - LAN: 1x 10/100M
 - LED: Wifi 3x blue. Programmable
 - Button: WPS

MAC addresses as verified by OEM firmware:

use address source
LAN *:83 factory 0xe000
2g *:85 factory 0x4
5g *:86 factory 0x8004

How to install:
1- Setup a TFTP server on a machine with IP address 192.168.1.10/24
2- Name the image as `firmware_auto.bin` and place it on the root of the
TFTP server
3- Connect the device via Ethernet, it should pick and flash the image

Signed-off-by: Rodrigo Araujo <araujo.rm@gmail.com>