Add PRM functions to test for pending PRM IRQs.  This will be used in
a subsequent patch to implement the PRM interrupt handler on the MPU.

Add PRM functions to ensure that all outstanding writes from the MPU
to the PRM IP block have completed before continuing execution.  This
will be used in a subsequent patch to ensure that all PRM interrupt
status bits are cleared in the hardware before exiting the ISR.
Normally we would not expose such a low-level function to other code.
But the current implementation of the PRM interrupt code, which uses
the generic IRQ chip code, doesn't give us a choice.

The pending PRM IRQ functions are based on code originally written by
Tero Kristo <t-kristo@ti.com>.
Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Tero Kristo <t-kristo@ti.com>