ARM: SAMSUNG: Ensure struct sys_device is declared in plat/pm.h

Previously we were relying on it being pulled in by other headers for
the prototype of s3c24xx_irq_suspend() and s3c24xx_irq_resume().

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>