iommu/omap: Mark pm functions __maybe_unused

The runtime_pm functions are unused when CONFIG_PM is disabled:

drivers/iommu/omap-iommu.c:1022:12: error: unused function 'omap_iommu_runtime_suspend' [-Werror,-Wunused-function]
static int omap_iommu_runtime_suspend(struct device *dev)
drivers/iommu/omap-iommu.c:1064:12: error: unused function 'omap_iommu_runtime_resume' [-Werror,-Wunused-function]
static int omap_iommu_runtime_resume(struct device *dev)

Mark them as __maybe_unused to let gcc silently drop them
instead of warning.

Fixes: db8918f6 ("iommu/omap: streamline enable/disable through runtime pm callbacks")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>