regulator: Add bindings for MT6331 regulator

Add the bindings for the regulators found in the MT6331 PMIC.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220913123456.384513-2-angelogioacchino.delregno@collabora.com


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