iommu/mediatek: Mark PM functions as __maybe_unused

When CONFIG_PM is unset, we get a harmless warning for this driver:

drivers/iommu/mtk_iommu.c:665:12: error: 'mtk_iommu_suspend' defined but not used [-Werror=unused-function]
drivers/iommu/mtk_iommu.c:680:12: error: 'mtk_iommu_resume' defined but not used [-Werror=unused-function]

Marking the functions as __maybe_unused gits rid of the two functions
and lets the compiler silently drop the object code, while still
doing syntax checking on them for build-time verification.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 0df4fabe ("iommu/mediatek: Add mt8173 IOMMU driver")
Signed-off-by: Joerg Roedel <jroedel@suse.de>