ARM: shmobile: smp: Make shmobile_smp_apmu_cpu_shutdown() static

shmobile_smp_apmu_cpu_shutdown() is used inside platsmp-apmu.c only.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>