regulator: pf8x00: add a doc for the module

pf8x00 module build was not documented.

Signed-off-by: Adrien Grassein <adrien.grassein@gmail.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Link: https://lore.kernel.org/r/20210114174714.122561-2-adrien.grassein@gmail.com


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