dt-bindings: mfd: Add bindings for the keys as subnode of PMIC

This patch adds documentation for device tree bindings for keys support
as the subnode of MT6397/MT6323 PMIC.
Signed-off-by: NChen Zhong <chen.zhong@mediatek.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NLee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/mt6397.txt

@@ -7,6 +7,7 @@ MT6397/MT6323 is a multifunction device with the following sub modules:
 - GPIO
 - Clock
 - LED
+- Keys
 
 It is interfaced to host controller using SPI interface by a proprietary hardware
 called PMIC wrapper or pwrap. MT6397/MT6323 MFD is a child device of pwrap.
@@ -40,6 +41,11 @@ Optional subnodes:
 		- compatible: "mediatek,mt6323-led"
 	see Documentation/devicetree/bindings/leds/leds-mt6323.txt
 
+- keys
+	Required properties:
+		- compatible: "mediatek,mt6397-keys" or "mediatek,mt6323-keys"
+	see Documentation/devicetree/bindings/input/mtk-pmic-keys.txt
+
 Example:
 	pwrap: pwrap@1000f000 {
 		compatible = "mediatek,mt8135-pwrap";