ARM: OMAP5: Reuse prm read_inst/write_inst

Make use of 'prm_base' so that  prm read_inst/write_inst can work on
OMAP5 devices.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>