ARM: SAMSUNG: Move common save/restore helpers to separate file

To separate legacy PM code from generic helpers, this patch moves the
generic register save/restore helpers to a new file called pm-common.c
that is compiled always when CONFIG_PM_SLEEP is enabled, to allow
platforms that do not want to use the legacy PM code use the generic
helpers.

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>