ARM: EXYNOS: Stop using legacy Samsung PM code

Since Exynos SoCs does not follow most of the semantics of older SoCs
when configuring the system to enter sleep, there is no reason to rely
on the legacy Samsung PM core anymore.

This patch adds local Exynos suspend ops and removes all the code left
unnecessary. As a side effect, suspend support on Exynos becomes
multiplatform-friendly.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>