ANDROID: KVM: arm64: iommu: Don't map assignable MMIO

We used to allow any MMIO to be mapped to the IOMMU without any
tracking as it was never assigned.

However now as we can assign it. we deny the host of mapping
MMIO of assigned devices in the IOMMU.
This is to restrictive as we should allow the host to map it if it
is currently owning the device.
But as MMIO doesn't exist in the vmemmap, it can't be refcounted
or tracked easily and as the only device I know that is mapped in
the IOMMU is the gic and that is never assigned we prevent mapping
MMIO that are assignable in the IOMMU, this can be relaxed later.

Signed-off-by: Mostafa Saleh <smostafa@google.com>