mips: ralink: mt7620: soc queries and tests as functions

Move the SoC register value queries and tests to specific functions,
to remove repetition of logic. No functional changes intended

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