dt-bindings: PCI: intel: Add YAML schemas for the PCIe RC controller

Add YAML schemas for PCIe RC controller on Intel Gateway SoCs
which is Synopsys DesignWare based PCIe core.

Signed-off-by: Dilip Kota <eswara.kota@linux.intel.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Andrew Murray <andrew.murray@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>