[x86][acpi] Have ACPI driver report a pdev instance for cpu_trace

This will let us route a BTI to cpu_trace so it can lookup physical
addresses. "ACPI" is a bit of a misnomer at this point, it's really
the platform bus driver for x86 systems.

Change-Id: I548b8932c89a964e65f8c3166af0347d267d67c7