MAINTAINERS: Add MFD's DT bindings directory to MFD entry

Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

MAINTAINERS

@@ -7945,6 +7945,7 @@ MULTIFUNCTION DEVICES (MFD)
 M:	Lee Jones <lee.jones@linaro.org>
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/lee/mfd.git
 S:	Supported
+F:	Documentation/devicetree/bindings/mfd/
 F:	drivers/mfd/
 F:	include/linux/mfd/