ARM: EXYNOS: Add support for firmware-assisted suspend/resume

On a numer of Exynos-based boards Linux kernel is running in non-secure
mode under a secure firmware. This means that certain operations need to
be handled in special way, with firmware assistance. System-wide
suspend/resume is an example of such operations.

This patch adds support for firmware-assisted suspend/resume by
leveraging recently introduced suspend and resume firmware operations
and modifying existing suspend/resume paths to account for presence of
secure firmware.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
[kgene.kim@samsung.com: rebased]
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>