ARM: firmware: add AFTR mode support to firmware do_idle method

On some platforms (i.e. EXYNOS ones) more than one idle mode is
available and we need to distinguish them in firmware do_idle method.

Add mode parameter to do_idle firmware method and AFTR mode support
to EXYNOS do_idle implementation.

This change is a preparation for adding secure firmware support to
EXYNOS cpuidle driver.

This patch shouldn't cause any functionality changes.

Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>