regulator: mt6397: Document valid modes

Document valid mode values for BUCK regulators.
Signed-off-by: Anand K Mistry <amistry@google.com>
Link: https://lore.kernel.org/r/20200702162231.v2.2.I0a814b246cfe47f8dd1e25553ee881cbcfd0d8eb@changeidSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/regulator/mt6397-regulator.txt

@@ -16,6 +16,9 @@ LDO:
   ldo_vemc3v3, ldo_vgp1, ldo_vgp2, ldo_vgp3, ldo_vgp4, ldo_vgp5, ldo_vgp6,
   ldo_vibr
 
+BUCK regulators can set regulator-initial-mode and regulator-allowed-modes to
+values specified in dt-bindings/regulator/mediatek,mt6397-regulator.h
+
 Example:
 	pmic {
 		compatible = "mediatek,mt6397";