The restore functions do not modify the passed in struct sleep_save,
so that parameter can be const.

This allows us to pass in const struct.  This allows us to use const
structs sleep_save to define system registers that will always be
restored to a constant value.
Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>