MAINTAINERS: Add file patterns for PCI 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: Bjorn Helgaas <bhelgaas@google.com>

MAINTAINERS

@@ -8678,6 +8678,7 @@ L:	linux-pci@vger.kernel.org
 Q:	http://patchwork.ozlabs.org/project/linux-pci/list/
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/helgaas/pci.git
 S:	Supported
+F:	Documentation/devicetree/bindings/pci/
 F:	Documentation/PCI/
 F:	drivers/pci/
 F:	include/linux/pci*