memory: tegra: Avoid unused function warnings

The suspend/resume functions have no callers depending on
configuration, so they must be marked __maybe_unused to
avoid these harmless warnings:

drivers/memory/tegra/tegra186.c:1578:12: error: 'tegra186_mc_resume' defined but not used [-Werror=unused-function]
 1578 | static int tegra186_mc_resume(struct device *dev)
      |            ^~~~~~~~~~~~~~~~~~
drivers/memory/tegra/tegra186.c:1573:12: error: 'tegra186_mc_suspend' defined but not used [-Werror=unused-function]
 1573 | static int tegra186_mc_suspend(struct device *dev)
      |            ^~~~~~~~~~~~~~~~~~~

Fixes: 177602b0 ("memory: tegra: Add system sleep support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>