PCI: dwc: dra7xx: Enable x2 mode support for dra74x, dra76x and dra72x

dra74x/dra76x and dra72x have separate compatible strings. Add support
for these compatible strings in pci-dra7xx driver to perform syscon
configurations required to get x2 mode working.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>