MAINTAINERS: Add file patterns for staging 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: linux-iio@vger.kernel.org
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

MAINTAINERS

@@ -10561,6 +10561,7 @@ STAGING - INDUSTRIAL IO
 M:	Jonathan Cameron <jic23@kernel.org>
 L:	linux-iio@vger.kernel.org
 S:	Odd Fixes
+F:	Documentation/devicetree/bindings/staging/iio/
 F:	drivers/staging/iio/
 
 STAGING - LIRC (LINUX INFRARED REMOTE CONTROL) DRIVERS