ath79: add support for Joy-IT JT-OR750i

ath79: add support for Joy-IT JT-OR750i

Specifications:
 * QCA9531, 16 MiB flash (Winbond W25Q128JVSQ), 128 MiB RAM
 * 802.11n 2T2R (external antennas)
 * QCA9887, 802.11ac 1T1R (connected with diplexer to one of the antennas)
 * 3x 10/100 LAN, 1x 10/100 WAN
 * UART header with pinout printed on PCB

Installation:
 * The device comes with a bootloader installed only
 * The bootloader offers DHCP and is reachable at http://10.123.123.1
 * Accept the agreement and flash sysupgrade.bin
 * Use Firefox if flashing does not work

TFTP recovery with static IP:
 * Rename sysupgrade.bin to jt-or750i_firmware.bin
 * Offer it via TFTP server at 192.168.0.66
 * Keep the reset button pressed for 4 seconds after connecting power

TFTP recovery with dynamic IP:
 * Rename sysupgrade.bin to jt-or750i_firmware.bin
 * Offer it via TFTP server with a DHCP server running at the same address
 * Keep the reset button pressed for 6 seconds after connecting power

Co-authored-by: Sebastian Schaper <openwrt@sebastianschaper.net>
Signed-off-by: Vincent Wiemann <vincent.wiemann@ironai.com>
(cherry picked from commit 55b4b3655263984b92e4b9fc515a5e6b8003c655)