CHROMIUM: ARM: exynos: pm: add one __maybe_unused annotation

CHROMIUM: ARM: exynos: pm: add one __maybe_unused annotation

It's hard to tell from the code flow (which is nasty in the first place) if the function will or will not be used, so
just mark it __maybe_unused for now.

Resolves:

arch/arm/mach-exynos/pm_domains.c:138:20: warning: 'exynos_pm_add_dev_to_genpd' defined but not used [-Wunused-function]

BUG=chromium-os:34287
TEST=none

Change-Id: Ieb8290edf595e33885b89eca29c83b460613a702
Signed-off-by: Olof Johansson <olofj@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/32837
Reviewed-by: Doug Anderson <dianders@chromium.org>