ARM: OMAP4: PRM: use the generic prm_inst to allow logic to be abstracted

use the generic function to pick up the prm_instance for a generic logic
which can be reused from OMAP4+

Signed-off-by: Nishanth Menon <nm@ti.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>