ARM: EXYNOS: Register cpuidle device only on exynos4210 and 5250

Currently, the exynos cpuidle driver works correctly only on exynos4210
and 5250. Trying to use it with just one CPU online on any other exynos
SoCs will lead to system failure, due to unsupported AFTR mode on other
SoCs. This patch fixes the problem by registering the driver only on
supported SoCs and letting others simply use default WFI mode until
support for them is added.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>