ARM: dts: DRA74x: Use PCIe compatible specific to dra74

Use dra74 specific compatible string for PCIe while preserving
the older compatible. While at that add a new label for the
2nd instance of pcie.

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>