SGI-Clark.Helios platform is based on multi-threaded CPUs and uses an
additional thread power domain level as well.

Define a power domain tree descriptor 'sgi_clark_helios_pd_tree_desc'
for SGI-Clark.Helios platform and let the function
'plat_get_power_domain_tree_desc' pick up the correct power
domain tree descriptor based on the platform.

Change-Id: Ibc6d551b570bc740053316a3608c455679d9155b
Signed-off-by: Chandni Cherukuri <chandni.cherukuri@arm.com>