MAINTAINERS: add devicetree bindings to Input Drivers section

To avoid changes to input bindings not reaching linux-input reviewers
add an appropriate file pattern to the MAINTAINERS entry.

Reported-by: Heiko Stübner <heiko@sntech.de>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>