spi: Add file patterns for spi 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>
Signed-off-by: Mark Brown <broonie@kernel.org>

MAINTAINERS

@@ -10777,6 +10777,7 @@ L:	linux-spi@vger.kernel.org
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/broonie/spi.git
 Q:	http://patchwork.kernel.org/project/spi-devel-general/list/
 S:	Maintained
+F:	Documentation/devicetree/bindings/spi/
 F:	Documentation/spi/
 F:	drivers/spi/
 F:	include/linux/spi/