ar71xx: add support for RB750r2

This patch adds support for the MikroTik RouterBOARD hEX lite
https://routerboard.com/RB750r2

Specifications:
- SoC: Qualcomm QCA9531 (850MHz)
- RAM: 64MB
- Storage: 16MB NOR SPI flash
- Ethernet: 5x100M (1 PoE in)

This ethernet router is based on the same platform as the hEX PoE lite (it
shares the same board identifier), but has no USB and no PoE out.

Signed-off-by: Thibaut VARENE <hacks@slashdirt.org>

target/linux/ar71xx/base-files/lib/upgrade/platform.sh

@@ -343,6 +343,7 @@ platform_check_image() {
 	ls-sr71|\
 	pb42|\
 	pb44|\
+	rb-750-r2|\
 	rb-750up-r2|\
 	rb-941-2nd|\
 	rb-951ui-2nd|\
@@ -618,6 +619,7 @@ platform_pre_upgrade() {
 	local board=$(ar71xx_board_name)
 
 	case "$board" in
+	rb-750-r2|\
 	rb-750up-r2|\
 	rb-941-2nd|\
 	rb-951ui-2nd|\
@@ -657,6 +659,7 @@ platform_do_upgrade() {
 	local board=$(ar71xx_board_name)
 
 	case "$board" in
+	rb-750-r2|\
 	rb-750up-r2|\
 	rb-941-2nd|\
 	rb-951ui-2nd|\