ARM: cpuidle: add const qualifier to cpuidle_ops member in structures

The core code does not modify smp_operations structures. To clarify it,
this patch adds 'const' qualifier to the 'ops' member of struct
of_cpuidle_method.

This change allows each arm cpuidle code to add 'const' qualifier to
its cpuidle_ops structure.

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