plat/arm/sgi: override weak implementation of plat_arm_get_cpu_pe_count

To support platforms which are based on multi-threaded CPUs, override
the weak implementation of plat_arm_get_cpu_pe_count function to return
the number of threads supported by the CPU used in the platform.

Change-Id: Ia680773f1277b17e2d3d2414d87943dcece33e89
Signed-off-by: Chandni Cherukuri <chandni.cherukuri@arm.com>