ramips: add RB750Gr3 native support

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>