arm64: kernel: Update kerneldoc for cpu_suspend() rename

Commit af391b15 ("arm64: kernel: rename __cpu_suspend to keep it
aligned with arm") renamed cpu_suspend() to arm_cpuidle_suspend(), but
forgot to update the kerneldoc header.

Fixes: af391b15

 ("arm64: kernel: rename __cpu_suspend to keep it aligned with arm")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Will Deacon <will.deacon@arm.com>