regulator: mp886x: implement set_ramp_delay

Implement the .set_ramp_delay for MP8867 and MP8869. MP8867 and MP8869
could share the implementation, the only difference is the slew_rates
array.

Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Link: https://lore.kernel.org/r/20200729130913.3ac38b32@xhacker


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