arm64: acpi: Move get_cpu_for_acpi_id() to a header

ACPI identifies CPUs by UID. get_cpu_for_acpi_id() maps the ACPI UID
to the linux CPU number.

The helper to retrieve this mapping is only available in arm64's numa
code.

Move it to live next to get_acpi_id_for_cpu().

Signed-off-by: James Morse <james.morse@arm.com>