dt-bindings: PCI: dra7xx: Add SoC specific compatible strings

Add new compatible strings for dra74x SoC (also used by dra76x) and
dra72x. This can be used to perform SoC specific configuration
(like configuring PCIe in x2 lane mode).
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/pci/ti-pci.txt

 TI PCI Controllers
 
 PCIe DesignWare Controller
- - compatible: Should be "ti,dra7-pcie" for RC
-	       Should be "ti,dra7-pcie-ep" for EP
+ - compatible: Should be "ti,dra7-pcie" for RC (deprecated)
+	       Should be "ti,dra7-pcie-ep" for EP (deprecated)
+	       Should be "ti,dra746-pcie-rc" for dra74x/dra76 in RC mode
+	       Should be "ti,dra746-pcie-ep" for dra74x/dra76 in EP mode
+	       Should be "ti,dra726-pcie-rc" for dra72x in RC mode
+	       Should be "ti,dra726-pcie-ep" for dra72x in EP mode
  - phys : list of PHY specifiers (used by generic PHY framework)
  - phy-names : must be "pcie-phy0", "pcie-phy1", "pcie-phyN".. based on the
 	       number of PHYs as specified in *phys* property.