mfd: arizona: Add comment to explain non-devm regulator_get

To avoid someone attempting to change this regulator_get back into a
devm_regulator_get put a comment in explaining that devres can't be used
here as the regulator will be destroyed before devres calls
regulator_put.

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