regulator: dt-bindings: Document the PM660/PM660L PMICs entries

The PM660 and PM660L combo is found on boards featuring the
SDM630, SDM636, SDM660 (and SDA variants) and it is used to
give power to practically everything, from core to peripherals.

Document the SMD-RPM regulator entries for both.

Signed-off-by: AngeloGioacchino Del Regno <kholk11@gmail.com>
Link: https://lore.kernel.org/r/20200926125549.13191-8-kholk11@gmail.com


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