ARM: exynos: exynos_pm_add_dev_to_genpd may be unused

exynos_pm_add_dev_to_genpd is used if one or more out of a large
number of Kconfig symbols are enabled. However the new
exynos_defconfig selects none of those, so the function becomes
unused. Marking it so lets the compiler automatically discard
it.

Without this patch, building exynos_defconfig results in:

arch/arm/mach-exynos/pm_domains.c:118:123: warning: 'exynos_pm_add_dev_to_genpd' defined but not used [-Wunused-function]

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Thomas Abraham <thomas.abraham@linaro.org>
Acked-by: Kukjin Kim <kgene.kim@samsung.com>
Cc: Rafael J. Wysocki <rjw@sisk.pl>