ARM: EXYNOS: Move the AFTR state function into pm.c

In order to remove depedency on pm code, let's move the 'exynos_enter_aftr'
function into the pm.c file as well as the other helper functions.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>