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: NJisheng Zhang <Jisheng.Zhang@synaptics.com>
Link: https://lore.kernel.org/r/20200729130913.3ac38b32@xhackerSigned-off-by: NMark Brown <broonie@kernel.org>