Documentation: devicetree: Add LED subnode binding for MT6323 PMIC

This patch adds documentation for devicetree bindings
for LED support as the subnode of MT6323 PMIC
Signed-off-by: NSean Wang <sean.wang@mediatek.com>
Signed-off-by: NLee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/mt6397.txt

@@ -34,6 +34,10 @@ Optional subnodes:
 - clk
 	Required properties:
 		- compatible: "mediatek,mt6397-clk"
+- led
+	Required properties:
+		- compatible: "mediatek,mt6323-led"
+	see Documentation/devicetree/bindings/leds/leds-mt6323.txt
 
 Example:
 	pwrap: pwrap@1000f000 {