[ARM] 5224/2: PXA SSP: Introduce register accessors

Introduce accessors for the SSP registers so that drivers don't need to
open code offsets from the MMIO base.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Eric Miao <eric.miao@marvell.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>