regulator: mp5416: add mp5416 regulator driver

Adding regulator driver for the device mp5416.
The MP5416 PMIC device contains four DC-DC buck converters and
five regulators, accessed over I2C.

Signed-off-by: Saravanan Sekar <sravanhome@gmail.com>
Link: https://lore.kernel.org/r/20200204110419.25933-3-sravanhome@gmail.com


Signed-off-by: Mark Brown <broonie@kernel.org>