amba: tegra-ahb: Mark PM functions as __maybe_unused

clang warns about an unused variable when CONFIG_PM is disabled, since
it is only referenced from an #ifdef:

drivers/amba/tegra-ahb.c:97:18: error: variable 'tegra_ahb_gizmo' is not needed and will not be emitted [-Werror,-Wunneeded-internal-declaration]

Rather than trying to get the #ifdef right, remove it and use
__maybe_unused here, which is less error prone.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Thierry Reding <treding@nvidia.com>