Add RT5120 PMIC regulator support.

It integrates 4 buck convertes, 1 LDO voltage regulator, 1 external
enable signal to control the external power source.
Signed-off-by: NChiYuan Huang <cy_huang@richtek.com>
Link: https://lore.kernel.org/r/1655892104-10874-4-git-send-email-u0084500@gmail.comSigned-off-by: NMark Brown <broonie@kernel.org>