ARM: SAMSUNG: Fix warning 's3c_pm_show_resume_irqs' defined but not used

s3c_pm_show_resume_irqs() is used by some s3c_pm_arch_show_resume_irqs()
implementations, which get included through mach/pm-core.h. Add __maybe_unused
to silence warnings when it isn't used (e.g. on S3C64XX platforms).

Signed-off-by: Maurus Cuelenaere <mcuelenaere@gmail.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>