regulator: mpq7920: add mpq7920 regulator driver

Adding regulator driver for the device mpq7920.
The MPQ7920 PMIC device contains four DC-DC buck converters and
five regulators, is designed for automotive and accessed over I2C.

Fixed sparse warning reported on this patch
Reported-by: kbuild test robot <lkp@intel.com>

Signed-off-by: Saravanan Sekar <sravanhome@gmail.com>
Link: https://lore.kernel.org/r/20200109195346.30270-1-sravanhome@gmail.com


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