drivers: regulator: npm1300: workaround for LDO HW bug

There is a HW bug in nPM1300 LDO which causes the LDO output voltage to
reach its target very slowly in specific cases. This is worked around by
performing an additional i2c read shortly after an LDO is enabled.

Signed-off-by: Sergei Ovchinnikov <sergei.ovchinnikov@nordicsemi.no>