regulator: mp8859: Support mode operations

The MP8859 provides mode control, implement the relevant regulator API
operations.

Tested-by: Markus Reichl <m.reichl@fivetechno.de>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://msgid.link/r/20240225-regulator-mp8859-v1-4-68ee2c839ded@kernel.org


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