plat/arm/sgi: override 'plat_psci_ops_t' for SGI-Clark.Helios platform

For SGI-Clark.Helios platform, at present, only the CPU power ON/OFF
ops are supported. So override the PSCI ops to allow callbacks only
for CPU power ON/OFF operations.

Change-Id: Idc0a3deb78cb850310cbe849d77604fa9881579c
Signed-off-by: Chandni Cherukuri <chandni.cherukuri@arm.com>