mips: ralink: mt7620: define MT7620_SYSC_BASE with __iomem

So that MT7620_SYSC_BASE can be used later in multiple functions without
needing to repeat this __iomem declaration each time

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