MAINTAINERS: Add file patterns for x86 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: Thomas Gleixner <tglx@linutronix.de>
Cc: "H . Peter Anvin" <hpa@zytor.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: devicetree@vger.kernel.org
Link: https://lkml.kernel.org/r/20180622100820.29616-1-geert@linux-m68k.org

MAINTAINERS

@@ -15366,6 +15366,7 @@ M:	x86@kernel.org
 L:	linux-kernel@vger.kernel.org
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git x86/core
 S:	Maintained
+F:	Documentation/devicetree/bindings/x86/
 F:	Documentation/x86/
 F:	arch/x86/