regulator: dt-bindings: Document the PM660/660L SPMI PMIC 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 SPMI regulator bindings for both.

Signed-off-by: AngeloGioacchino Del Regno <kholk11@gmail.com>
Link: https://lore.kernel.org/r/20200926125549.13191-5-kholk11@gmail.com


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