iommu: Disable on !MMU builds

A lot of the IOMMU support code does not build if the CPU does
not have an MMU itself, and it's not clear if there is any
use case for it, so let's just disable it and wait for anybody
to need it.

This avoids randconfig errors like

../arch/arm/mm/dma-mapping.c: In function '__iommu_alloc_remap':
../arch/arm/mm/dma-mapping.c:1278:34: error: 'VM_ARM_DMA_CONSISTENT' undeclared (first use in this function)
  area = get_vm_area_caller(size, VM_ARM_DMA_CONSISTENT | VM_USERMAP,
                                  ^
../arch/arm/mm/dma-mapping.c:1278:34: note: each undeclared identifier is reported only once for each function it appears in
../arch/arm/mm/dma-mapping.c: In function '__atomic_get_pages':
../arch/arm/mm/dma-mapping.c:1358:27: error: 'atomic_pool' undeclared (first use in this function)
  struct dma_pool *pool = &atomic_pool;

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Joerg Roedel <jroedel@suse.de>