dt-bindings: mfd: mt6397: Add binding for MT6357

Add binding documentation for the MT6357 PMIC.

Signed-off-by: Fabien Parent <fparent@baylibre.com>
Signed-off-by: Alexandre Mergnat <amergnat@baylibre.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20221005-mt6357-support-v3-1-7e0bd7c315b2@baylibre.com