ARM: dts: DRA72x: Use PCIe compatible specific to dra72

Use dra72 specific compatible string for PCIe while preserving
the older compatible.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>