regulator: Fairchild fan53555 support

This driver supports Fairchild FAN53555 Digitally Programmable
TinyBuck Regulator. The FAN53555 is a step-down switching voltage
regulator that delivers a digitally programmable output from an
input voltage supply of 2.5V to 5.5V. The output voltage is
programmed through an I2C interface.

Signed-off-by: Yunfan Zhang <yfzhang@marvell.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>