drivers/rtc/rtc-88pm80x.c: add CONFIG_PM_SLEEP to suspend/resume functions

Add CONFIG_PM_SLEEP to suspend/resume functions to fix the following
build warning when CONFIG_PM_SLEEP is not selected.  This is because
sleep PM callbacks defined by SIMPLE_DEV_PM_OPS are only used when the
CONFIG_PM_SLEEP is enabled.

  drivers/rtc/rtc-88pm80x.c:238:12: warning: 'pm80x_rtc_suspend' defined but not used [-Wunused-function]
  drivers/rtc/rtc-88pm80x.c:243:12: warning: 'pm80x_rtc_resume' defined but not used [-Wunused-function]

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>