ARM: EXYNOS: Remove left over 'extra_save'

Since 32b0aa9a ("ARM: EXYNOS: Remove i2c sys configuration related
code") the Exynos 5250 no longer saves additional registers under
'exynos_pm_data.extra_save' field.

No one else uses this code so get rid of it making also 'exynos_pm_data'
const everywhere.

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>