ramips: add RB750Gr3 native support

This patch adds support of MikroTik RouterBOARD 750Gr3, without the need
to reflashing the bootloader.

Installation through RouterBoot follows the usual MikroTik method
https://openwrt.org/toh/mikrotik/common

Since the image isn't compatible with RouterBOARD 750Gr3 installations
which have replaced the bootloader, the former used userspace boardname
is not added to the SUPPORTED_DEVICES, to prevent a brick while trying
to upgrade to the image with native support.

Signed-off-by: Anton Arapov <arapov@gmail.com>
Signed-off-by: Thibaut VARÈNE <hacks@slashdirt.org>
Signed-off-by: Mathias Kresin <dev@kresin.me>

target/linux/ramips/base-files/lib/ramips.sh

@@ -415,9 +415,6 @@ ramips_board_detect() {
 	*"R6220")
 		name="r6220"
 		;;
-	*"RB750Gr3")
-		name="rb750gr3"
-		;;
 	*"RE350 v1")
 		name="re350-v1"
 		;;