ARM: EXYNOS: Fix build with ARM_CPU_SUSPEND=n

"ARM: EXYNOS: Add support for firmware-assisted suspend/resume" patch
added to arch/arm/mach-exynos/firmware.c new references to functions
from arch/arm/mach-exynos/sleep.S causing the new CONFIG_PM_SLEEP=n
build breakages.  Then "ARM: EXYNOS: Fix build with PM_SLEEP=n and
ARM_EXYNOS_CPUIDLE=y" patch tried to fix the CONFIG_PM_SLEEP=n issues
by always building sleep.S which caused the CONFIG_ARM_CPU_SUSPEND=n
build breakage.  Fix it by building arch/arm/mach-exynos/sleep.o only
for CONFIG_EXYNOS_CPU_SUSPEND=y and adding appropriate IS_ENABLED()
checks to arch/arm/mach-exynos/firmware.c.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
[b.zolnierkie: fixed ->resume check and added patch description]
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>