ARM: EXYNOS: Annotate iomem and pm_data pointers __ro_after_init

The pointers to __iomem sysram and exynos_pm_data are set only during
initcalls.  Later the pointers itself are used only in read-only way so
we can mark them __ro_after_init to increase code safeness.

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