regulator: mp886x: use "mps,switch-frequency-hz"

As Rob suggested, use the "mps,switch-frequency-hz" instead of the
"mps,switch-frequency" for switch frequency. Fortunately, the switch
frequency support isn't released, so we can modify it now without
any concern.

Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Link: https://lore.kernel.org/r/20200824102402.4047fa5f@xhacker.debian


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