mfd: arizona: Factor out read of device tree GPIOs

This patch factors out the reading of GPIOs for the Arizona devices
into a helper function.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>