regulator: Add document for MT6358 regulator

add dt-binding document for MediaTek MT6358 PMIC

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Hsin-Hsiung Wang <hsin-hsiung.wang@mediatek.com>
Link: https://lore.kernel.org/r/1566531931-9772-6-git-send-email-hsin-hsiung.wang@mediatek.com


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