ARM: s3c64xx: Constify wake_irqs

samsung_sync_wakemask() accepts pointer to const data so wake_irqs can
be made const to increase safeness.

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