regulator: mp8859: Specify register accessibility and enable caching

Avoid needless accesses to the hardware by caching register values that
we know, marking status registers as volatile as appropriate.

Tested-by: Markus Reichl <m.reichl@fivetechno.de>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://msgid.link/r/20240225-regulator-mp8859-v1-1-68ee2c839ded@kernel.org


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