ARM: dts: dra7: Remove deprecated PCI compatible string

Since SoC specific compatible strings are used for PCIe in
dra74x.dtsi and dra72x.dtsi, remove the compatible property
from dra7.dtsi which uses the deprecated compatible string.

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>