mips: dts: ralink: mt7628a: update system controller node and its consumers

Current MT7628A device tree file system controller node is wrong since it is
not matching bindings. Hence, update it to match current bindings updating
it also to use new introduced clock constants.

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