ARM: EXYNOS: Remove duplicated define of SLEEP_MAGIC

The magic cookie for entering sleep state was defined and used in
two different places: firmware.c and suspend.c. Move it to one common
place to reduce duplication.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski.k@gmail.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>