mips: ralink: introduce commonly used remap node function

The ralink_of_remap() function is repeated several times on SoC specific
source files. They have the same structure, but just differ in compatible
strings. In order to make commonly use of these codes, this patch
introduces a newly designed mtmips_of_remap_node() function to match and
remap all supported system controller and memory controller nodes.

Build and run tested on MT7620 and MT7628.

Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Reviewed-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>