ARM: EXYNOS: Constify exynos_pm_data array

The 'exynos5420_pm_data' is not modified and can be made const.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Pankaj Dubey <pankaj.dubey@samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>