ARM: EXYNOS: add support get_core_count() for EXYNOS5250

The EXYNOS5250 has two Cortex-A15 cores and there's no
need to call scu_enable() in platform_smp_prepare_cpus()
because Cortex-A15 has no regarding scu register which
can be used for getting number of cores.

Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>