regulator: stm32-pwr: Remove unneeded .min_uV and .list_volage

For fixed regulator, setting .n_voltages = 1 and .fixed_uV is enough,
no need to set .min_uV and .list_volage.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@kernel.org>