ARM: EXYNOS: Remove unneeded __ref annotation for cpu_die function

The __ref annotation for exynos_cpu_die() is not needed because the
function does not reference any __init/__exit symbol or call.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>