Update the parsing of the cpu node. We are unable to use the reg property
as the cpu id on arm64 as it may use two cells. In it's place we can use
the device id.

It is expected we will use the reg data on arm64 to enable cores so we
still need to read and store it even if it is not yet used.

Differential Revision: https://reviews.freebsd.org/D1555
Reviewed by: nwhitehorn
Sponsored by: The FreeBSD Foundation