feat(plat/arm/css): add per-cpu power down support for warm reset

Add a new function to setup a SGI interrupt that will be used to trigger
a request for per-cpu power down when executing the PSCI SYSTEM_RESET
request. This will be used on CSS platform that require all the CPUs to
execute the CPU specific power down sequence to complete a warm reboot
sequence in which only the CPUs are power cycled.

Change-Id: I80da0f6c3cd0c5c442c82239ba1e1f773821a7f5
Signed-off-by: Pranav Madhu <pranav.madhu@arm.com>