arm: exynos: Include missing CPU header in soc.c

samsung_get_base_swreset() is called in soc.c, but corresponding header
with its prototype is not included. Fix this to avoid possible build
errors.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>