regulator: mp8859: Support enable control

The MP8859 provides a software enable control, support it in the
regulator driver.

Tested-by: Markus Reichl <m.reichl@fivetechno.de>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://msgid.link/r/20240225-regulator-mp8859-v1-3-68ee2c839ded@kernel.org


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