mfd : Check if there are pending irq on the db8500 gic

This patch introduces a routine to check if there are some
irqs pending on the gic. Usually this check is not relevant because
it appears racy (an irq can arrive right after this check), but in
the ux500 it makes sense because the prcmu decouples the gic from
the A9 cores.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>