ramips: add support for Edimax BR-6478AC v2

ramips: add support for Edimax BR-6478AC v2

Roll-up of patches by Rohan Murch, Hans Ulli Kroll, and James McKenzie.
Taken from https://forum.openwrt.org/viewtopic.php?id=67192 and updated.

Specification:
- System-On-Chip: MT7620A
- CPU/Speed: 580 MHz
- Flash-Chip: Macronix MX25L6405D
- Flash size: 8192 KiB
- RAM: 64 MiB
- Wireless No1: SoC-integrated: MT7620A 2.4GHz 802.11bgn
- Wireless No2: On-board chip: MT7612E 5GHz 802.11ac
- Switch: Mediatek MT7530W Gigabit Switch
- USB: Yes 1 x 2.0

Installation:
1. Download sysupgrade.bin
2. Open vendor web interface
3. Choose to upgrade firmware
4. After reboot connect via ethernet at 192.168.1.1

Signed-off-by: Daniel Gimpelevich <daniel@gimpelevich.san-francisco.ca.us>
(backported from f72fa883b324311c1972c39ed521e536b25d674c)