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>