arm64: cpuinfo: make machine_name as extern

of_flat_dt_get_machine_name() API is marked as __init so
machine_name should be made as an extern in-order to get
it accessed by the cpuinfo.c. In the earlier kernel revisions
the usage was restricted to the one file setup.c only and
due to which we didn't faced any issue.

Change-Id: I70da7dcd45f7d94866e77a93c5a8ceb8866f3240
Signed-off-by: Trilok Soni <tsoni@codeaurora.org>
Signed-off-by: Channagoud Kadabi <ckadabi@codeaurora.org>