mmc: sunxi: mark PM functions as __maybe_unused

The newly added runtime-pm functions cause a harmless warning
when CONFIG_PM is disabled:

drivers/mmc/host/sunxi-mmc.c:1452:12: error: 'sunxi_mmc_runtime_suspend' defined but not used [-Werror=unused-function]
 static int sunxi_mmc_runtime_suspend(struct device *dev)
            ^~~~~~~~~~~~~~~~~~~~~~~~~
drivers/mmc/host/sunxi-mmc.c:1435:12: error: 'sunxi_mmc_runtime_resume' defined but not used [-Werror=unused-function]
 static int sunxi_mmc_runtime_resume(struct device *dev)

This marks them as __maybe_unused to shut up the warning.

Fixes: 9a8e1e8c ("mmc: sunxi: Add runtime_pm support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>