ath79: add support for MikroTik RouterBOARD 750 r2 (hEX lite)

ath79: add support for MikroTik RouterBOARD 750 r2 (hEX lite)

This patch adds support for the MikroTik RouterBOARD 750 r2, marketed as
hEX lite, a small indoor router with 5x 10/100 Mbps Ethernet ports, one
with PoE in. The device was already supported by the ar71xx target.

Specifications:
 - SoC: Qualcomm Atheros QCA9533
 - Flash: 16 MB SPI NOR
 - RAM: 64 MB
 - Ethernet: 4x 10/100 Mbps LAN, 1x 10/100 Mbps WAN (PoE in)
 - LEDs: 5x Ethernet port activity (green), 1x user (green)
 - Buttons: 1x reset

 See https://mikrotik.com/product/RB750r2 for more details.

Not working:
 - Serial port (already not working in ar71xx)

Flashing:
 TFTP boot initramfs image and then perform sysupgrade. Only the
 "Internet" port will ask for an initramfs image. Follow common
 MikroTik procedure as in https://openwrt.org/toh/mikrotik/common.

Signed-off-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>