regulator: rt4801: Add support for RT4801 Display Bias regulator driver

Adds support for the RT4801 DSV. It has two regulators (DSVP/DSVN) with
an I2C interface. DSVP/DSVN can provide the display panel module for the
positive/negative voltage range from (+/-)4V to (+/-)6V.

Signed-off-by: ChiYuan Huang <cy_huang@richtek.com>
Link: https://lore.kernel.org/r/1597461262-25878-1-git-send-email-u0084500@gmail.com


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