ARM: EXYNOS: Constify list of retention registers

The list of retention registers (release_ret_regs field of struct
exynos_pm_data and arrays with values) are not modified and can be made
const to improve the const safeness.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>