regulator: mp886x: support mps,switch-frequency

Both MP8867 and MP8869 support different switch frequency.
Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Link: https://lore.kernel.org/r/20200729130952.260671f1@xhackerSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/regulator/mp886x.txt

@@ -9,6 +9,10 @@ Required properties:
 - mps,fb-voltage-divider: An array of two integers containing the resistor
   values R1 and R2 of the feedback voltage divider in kilo ohms.
 
+Optional properties:
+- mps,switch-frequency: The valid switch frequency in Hertz. Available values
+  are: 500000, 750000, 1000000, 1250000, 1500000
+
 Any property defined as part of the core regulator binding, defined in
 ./regulator.txt, can also be used.