[media] atmel-isi: Protect PM-only functions to kill warning

If CONFIG_PM=n:

    drivers/media/platform/soc_camera/atmel-isi.c:1044: warning: 'atmel_isi_runtime_suspend' defined but not used
    drivers/media/platform/soc_camera/atmel-isi.c:1054: warning: 'atmel_isi_runtime_resume' defined but not used

Protect the unused functions by #ifdef CONFIG_PM to fix this.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>