arm64: show present cpu instead of online cpu in /proc/cpuinfo

Some userspace applications use /proc/cpuinfo to determine how many CPUs
the system has. CPU hotplug can offline a CPU at runtime and causing the
offline CPU not present in /proc/cpuinfo if we only show online cpu in
/proc/cpuinfo. Show all present CPUs in /proc/cpuinfo so that userspace
can take advantage of all availabie CPUs, even though some CPUs may have
been brought offline due to inactivity.

Change-Id: I6afe11e406c4a6ac0dd04d6abac11c2b68b9685a
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>