regulator: mp8859: Report slew rate

The MP8859 implements voltage changes at the rate of 1mV/us, tell the core
about this so that it can provide appropriate delays on voltage changes.

Tested-by: Markus Reichl <m.reichl@fivetechno.de>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://msgid.link/r/20240225-regulator-mp8859-v1-7-68ee2c839ded@kernel.org


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