MAINTAINERS: Add file patterns for rcar PCI device tree bindings

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

Link: https://lore.kernel.org/r/1588854799-13710-9-git-send-email-prabhakar.mahadev-lad.rj@bp.renesas.com


Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>