dt-bindings: mfd: Add the description for LED as the sub module

This patch adds description for LED as the sub-module on MT6397/MT6323
multifunction device.
Signed-off-by: NSean Wang <sean.wang@mediatek.com>
Signed-off-by: NLee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/mt6397.txt

@@ -6,6 +6,7 @@ MT6397/MT6323 is a multifunction device with the following sub modules:
 - Audio codec
 - GPIO
 - Clock
+- LED
 
 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.