MAINTAINERS: Add devicetree binding to PCI i.MX6 entry

Add an entry for the devicetree binding file, so that when people run
./scripts/get_maintainer.pl the PCI imx6 maintainers could also be listed.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Lucas Stach <l.stach@pengutronix.de>