mips: ralink: rt288x: define RT2880_SYSC_BASE with __iomem

 So that RT2880_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>