While handling PRCM IRQs, mask out interrupts that are not enabled in
PRM_IRQENABLE_MPU.  If these are not masked out, non-enabled
interrupts are caught, a WARN() is printed due to no 'handler' and the
events are cleared.  In addition to being noisy, this can also
interfere with independent polling of this register by SR/VP code.

This was noticed using SmartReflex transitions which cause the VPx_*
interrupts to be handled since they are set in PRM_IRQSTATUS_MPU even
but not enabled in PRM_IRQENABLE_MPU.
Acked-by: Mike Turquette <mturquette@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>