regulator: 88pm800: add regulator driver for 88pm800

Add the regulator driver for PMIC 88pm800 including device tree
support.
88pm800 is an I2C-based power-management IC containing voltage
regulators, a real-time clock, and some general purpose ADC devices,

Signed-off-by: Yi Zhang <yizhang@marvell.com>
Signed-off-by: Chao Xie <chao.xie@marvell.com>
Signed-off-by: Mark Brown <broonie@linaro.org>