ARM: exynos: correct kernel doc in platsmp

Correct few kerneldoc issues, including W=1 compile warning:

  arch/arm/mach-exynos/platsmp.c:89: warning:
    expecting prototype for exynos_core_power_down(). Prototype was for exynos_cpu_power_down() instead

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/20210405181605.52612-1-krzysztof.kozlowski@canonical.com