iommu: Make some functions static

The sparse tool complains as follows:

drivers/iommu/iommu.c:386:5: warning:
 symbol 'iommu_insert_resv_region' was not declared. Should it be static?
drivers/iommu/iommu.c:2182:5: warning:
 symbol '__iommu_map' was not declared. Should it be static?

Those functions are not used outside of iommu.c, so mark them static.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Link: https://lore.kernel.org/r/20200713142542.50294-1-weiyongjun1@huawei.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>