ARM: OMAP4: PRM: Remove hardcoding of PRM_IO_PMCTRL_OFFSET register

PRM_IO_PMCTRL_OFFSET need not be same for all SOCs hence
remove hardcoding and use the value provided by the omap_prcm_irq_setup
structure. This is done to support IO wakeup on am437x series.

Signed-off-by: Keerthy <j-keerthy@ti.com>
Reviewed-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>