MAINTAINERS: Add file patterns for serio device tree bindings

Submitters of device tree binding documentation may forget to CC
the subsystem maintainer if this is missing.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

MAINTAINERS

@@ -7005,6 +7005,7 @@ F:	include/uapi/linux/input.h
 F:	include/uapi/linux/input-event-codes.h
 F:	include/linux/input/
 F:	Documentation/devicetree/bindings/input/
+F:	Documentation/devicetree/bindings/serio/
 F:	Documentation/input/
 
 INPUT MULTITOUCH (MT) PROTOCOL