mfd: arizona: Factor out register polling

Factor out the polling of the interrupt status register whilst we wait
for boot done to allow the polling to be reused in other situations.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>