arm64: kernel: add missing __init section marker to cpu_suspend_init

Suspend init function must be marked as __init, since it is not needed
after the kernel has booted. This patch moves the cpu_suspend_init()
function to the __init section.

Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>