mailbox: tegra-hsp: mark suspend function as __maybe_unused

We get a warning about this being unused when CONFIG_PM is disabled:

drivers/mailbox/tegra-hsp.c:782:12: error: 'tegra_hsp_resume' defined but not used [-Werror=unused-function]

Fixes: 9a63f0f4

 ("mailbox: tegra-hsp: Add suspend/resume support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>