ARM: cpuidle: constify return value of arm_cpuidle_get_ops()

arm_cpuidle_read_ops() just copies '*ops' to cpuidle_ops[cpu], so the
structure '*ops' is not modified at all.

The comment is also updated accordingly.

Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>