sm501: Do not clear read only bits when writing registers

sm501: Do not clear read only bits when writing registers

When writing registers that have read only bits we have to avoid
changing these bits as they may have non zero values. Make sure we use
the correct masks to mask out read only and reserved bits when
changing registers.

Also remove extra spaces from dram_control and arbitration_control
assignments.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>