drivers: firmware: psci: use const and __initconst for psci_cpuidle_ops

The psci_cpuidle_ops structures is not over-written, so add "const"
qualifier and replace __initdata with __initconst.

Acked-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>