a5ds: Add handler for when user tries to switch off secondary cores

a5ds: Add handler for when user tries to switch off secondary cores

a5ds only has always-on power domain and there is no power control
present. However, without the pwr_domain_off handler, the kernel
panics when the user will try to switch off secondary cores. The
a5ds_pwr_domain_off handler will prevent kernel from crashing,
i.e. the kernel will attempt but fail to shut down the secondary CPUs
if the user tries to switch them offline.

Change-Id: I3c2239a1b6f035113ddbdda063c8495000cbe30c
Signed-off-by: Usama Arif <usama.arif@arm.com>