dt-bindings: power: Add domain regulator supply

Some power domains (eg. mfg) needs to turn on power supply before power
on.

Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Link: https://lore.kernel.org/r/20210129101208.2625249-2-hsinyi@chromium.org


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>