exynos: Avoid duplicate reset_cpu with SYSRESET enabled

The sysreset uclass unconditionally provides a definition of the
reset_cpu() function. So does the exynos soc code. Fix the build with
SYSRESET enabled by omitting the function from the soc code in that
case. The code still needs to be kept around for use in SPL.

This commit was inspired by commit 6e19dc84 ("sunxi: Avoid duplicate
reset_cpu with SYSRESET enabled").

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