ARM: EXYNOS: Constify local exynos_pmu_data structure

The Exynos5420 instance of exynos_pmu_data structure is not modified and
can be made const.

Suggested-by: Pavel Fedin <p.fedin@samsung.com>
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Pankaj Dubey <pankaj.dubey@samsung.com>