regulator: rt4801: Add DT binding documentation

Add a devicetree binding documentation for the rt4801 regulator driver.

Signed-off-by: ChiYuan Huang <cy_huang@richtek.com>
Link: https://lore.kernel.org/r/1597461262-25878-2-git-send-email-u0084500@gmail.com


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