MAINTAINERS: Add file patterns for iio 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: Jonathan Cameron <jic23@kernel.org>
Cc: Hartmut Knaack <knaack.h@gmx.de>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Cc: linux-iio@vger.kernel.org
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

MAINTAINERS

@@ -5541,6 +5541,7 @@ R:	Lars-Peter Clausen <lars@metafoo.de>
 R:	Peter Meerwald-Stadler <pmeerw@pmeerw.net>
 L:	linux-iio@vger.kernel.org
 S:	Maintained
+F:	Documentation/devicetree/bindings/iio/
 F:	drivers/iio/
 F:	drivers/staging/iio/
 F:	include/linux/iio/