regulator: of: Use of_property_read_u32() for reading min/max

OF interface provides to read the u32 value via standard interface
of_property_read_u32(). Use this API to read "regulator-min-microvolts"
and "regulator-max-microvolt".

This will make consistent with other property value reads.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Mark Brown <broonie@kernel.org>