MFC r303100 by andrew:

We will be switching to a new arm64 uart cpu driver that handles both FDT
and ACPI. As such pull out what will be the common parts of the FDT cpu
detection to a new function that can be shared between them.

Reviewed by: manu
Obtained from: ABT Systems Ltd
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D7262