ARM: exynos: Move Exynos542x CPU state reset to pm_prepare()

Reset the CPU state entry in sysram in pm_prepare() callback. That entry
doesn't need to be reset for every suspended CPU, so make it symmetic
with the code, which restores it in pm_resume() callback.

Tested on Samsung Chromebook2 Pit (Exynos5420) and Pi (Exynos5800)
boards.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>