regulator: add new lp8788 regulator driver

TI LP8788 PMU has 4 BUCKS and 22 LDOs.
The voltage of BUCK1 and BUCK2 can be controlled by external gpios.
And some LDOs also can be enabled by external gpios.
The regmap interface is used for regulator operations.

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>