regulator: mt6311: Add document for mt6311 regulator

This patch adds a list of supported regulator names to the devicetree
binding documentation for Mediatek MT6311 PMIC.

Signed-off-by: Henry Chen <henryc.chen@mediatek.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Mark Brown <broonie@kernel.org>