ARM: AM43xx: Add the PRM IRQ register offsets

Add the PRM IRQ register offsets.  This is needed to support PRM I/O
wakeup on AM43xx.

Signed-off-by: Keerthy <j-keerthy@ti.com>
[paul@pwsan.com: improved patch description, moved the PRM_IO_PMCTRL macro
 out of the CM section]
Reviewed-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>