ath79: add support for MikroTik RouterBOARD 960PGS

ath79: add support for MikroTik RouterBOARD 960PGS

This patch adds support for the MikroTik RouterBOARD 960PGS (hEX
PoE/PowerBox Pro) router. The device has a USB 2.0 port and an SFP port for
adding optical fiber connectivity. The ports 2-5 can power other PoE
capable devices with the same voltage as applied to the unit.

Specifications:

- SoC: Qualcomm Atheros QCA9557
- Flash: 16 MB (SPI)
- RAM: 128 MB
- 1x Ethernet SFP: 1000
- 1x Ethernet RJ45: 10/100/1000 port with passive POE in
- 4x Ethernet RJ45: 10/100/1000 ports with 802.3af/at PoE out
- 1x USB 2.0 host port
- 1x reset button

See [1] and [2] for more details.

Flashing:
TFTP boot initramfs image and then perform sysupgrade. Follow common
MikroTik procedure as in https://openwrt.org/toh/mikrotik/common.

Link: https://mikrotik.com/product/RB960PGS
Link: https://mikrotik.com/product/RB960PGS-PB
Signed-off-by: Oskari Lemmela <oskari@lemmela.net>
Signed-off-by: Chester A. Unal <chester.a.unal@arinc9.com>