ARM: S3C64XX: Fix build error with CONFIG_S3C_DEV_FB disabled

If there is no board selecting CONFIG_S3C_DEV_FB enabled, build will
fail on arch/arm/mach-s3c64xx/pm.c, where s3c_device_fb is referenced.

This patch adds ifdef guard around the code making it compile only
when CONFIG_S3C_DEV_FB is enabled.

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>