regulator: document binding for MT6315 regulator

Add device tree binding information for MT6315 regulator driver.
Example bindings for MT6315 are added.

Signed-off-by: Hsin-Hsiung Wang <hsin-hsiung.wang@mediatek.com>
Link: https://lore.kernel.org/r/1612678457-11548-2-git-send-email-hsin-hsiung.wang@mediatek.com


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