MAINTAINERS: Add file patterns for rng 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: Matt Mackall <mpm@selenic.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: linux-crypto@vger.kernel.org
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

MAINTAINERS

@@ -5259,6 +5259,7 @@ M:	Matt Mackall <mpm@selenic.com>
 M:	Herbert Xu <herbert@gondor.apana.org.au>
 L:	linux-crypto@vger.kernel.org
 S:	Odd fixes
+F:	Documentation/devicetree/bindings/rng/
 F:	Documentation/hw_random.txt
 F:	drivers/char/hw_random/
 F:	include/linux/hw_random.h