MAINTAINERS: Add file patterns for ata 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: Tejun Heo <tj@kernel.org>
Cc: linux-ide@vger.kernel.org
Signed-off-by: Tejun Heo <tj@kernel.org>