dt-bindings: regulator: Add document for MT6359 regulator

add dt-binding document for MediaTek MT6359 PMIC

Signed-off-by: Hsin-Hsiung Wang <hsin-hsiung.wang@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>