a5ds: Hold the secondary cpus in pen rather than panic

For the secondary CPUs, hold the cpu in wfe rather then panic.
This will be needed when multicore support is added to a5ds as
the smc call will write to the hold base and signal an event to
power on the secondary CPUs.

Change-Id: I0ffc2059e9ef894c21375ca5c94def859bfa6599
Signed-off-by: Usama Arif <usama.arif@arm.com>