ARM: dts: list the CPU nodes for EXYNOS5250

Instead of having to specify the number for CPUs in EXYNOS5250 in
platsmp.c file, let the number of CPUs be determined by having this
information listed in EXYNOS5250 device tree file.

Signed-off-by: Chander Kashyap <chander.kashyap@linaro.org>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>