mfd: rt5120: Add Richtek PMIC support

Add Richtek RT5120 PMIC I2C driver.

Signed-off-by: ChiYuan Huang <cy_huang@richtek.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/1660100142-32493-3-git-send-email-u0084500@gmail.com