MLK-22304-2 fbdev: mxsfb: fix build warnings if CONFIG_PM_SLEEP off

MLK-22304-2 fbdev: mxsfb: fix build warnings if CONFIG_PM_SLEEP off

The system suspend and resume hooks is related with CONFIG_PM_SLEEP
instead of CONFIG_PM, so use CONFIG_PM_SLEEP to control the definitions
for them. And after this, the related overlay suspend and resume should
be defined under CONFIG_PM_SLEEP control. Below are the build warnings
before this patch.

drivers/video/fbdev/mxsfb.c:2463:12: warning: ‘mxsfb_resume’ defined but not used [-Wunused-function]
 static int mxsfb_resume(struct device *pdev)
            ^~~~~~~~~~~~
drivers/video/fbdev/mxsfb.c:2444:12: warning: ‘mxsfb_suspend’ defined but not used [-Wunused-function]
 static int mxsfb_suspend(struct device *pdev)
            ^~~~~~~~~~~~~

Signed-off-by: Fancy Fang <chen.fang@nxp.com>