PCI: Add DT binding for "reset-gpios" property

Add DT binding for "reset-gpios" property which supports GPIO based PERST#
signal.

Signed-off-by: Manikanta Maddireddy <mmaddireddy@nvidia.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Thierry Reding <treding@nvidia.com>