MAINTAINERS: Add file patterns for serial 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>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-serial@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

MAINTAINERS

@@ -9996,6 +9996,7 @@ SERIAL DRIVERS
 M:	Greg Kroah-Hartman <gregkh@linuxfoundation.org>
 L:	linux-serial@vger.kernel.org
 S:	Maintained
+F:	Documentation/devicetree/bindings/serial/
 F:	drivers/tty/serial/
 
 SYNOPSYS DESIGNWARE DMAC DRIVER