ar71xx: add support for UniFi-AC-Mesh

This adds the build option for the new UniFi AC Mesh.
It is a direct hardware copy from the AC Lite.

- SoC: QCA9563-AL3A (775Mhz)
- RAM: 128MiB
- Flash: 16MiB - dual firmware partitions!
- LAN: 1 1000M - POE
- Wireless:
        2.4G: QCA9563
          5G: UniFi Chip, QCA988X compatible

Thanks to Frank Dietz for testing.

Signed-off-by: Ludwig Thomeczek <ledesrc@wxorx.net>
[wrapped too long lines in mach-ubnt-unifiac.c]
Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>