arm64: cpuidle: add __init section marker to arm_cpuidle_init

It is not needed after booting, this patch moves the arm_cpuidle_init()
function to the __init section.

Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>