OMAP4: PM: Fix missed PRCM MPU interrupt events

OMAP PRCM detects events with EDGE sensitivity. Edge sensitivity
requires to clear the interrupt event status bit before acknowledging
the source event.
If the source event is cleared (1) before reseting the interrupt event
status bit (2), then a subsequent source event (i.e edge) between (1)
and (2) would be missed. Then, all the source events would be kept
"masked" until the source event is cleared again.

The bug seen here is that upon two consecutive PRCM interrupt events
from the IO daisy chain source event, the second event is missed if it
occurs before the PRCM interrupt event status is cleared.
Since the IO daisy chain source event has not been acknowledged, the
subseqent events from this source are missed too.

This patch fix the PRCM interrupt handler by acknowledging the interrupt
event status bit before clearing the source event.

Change-Id: I8c43a93c3b6c7c654ddaa262b7c1c3e7385da158
Signed-off-by: Patrick Combes <p-combes@ti.com>
Signed-off-by: Cedric Poignet <c-poignet@ti.com>