ANDROID: KVM: arm64: Map guest MMIO as device memory

As guests use FWB, assignable MMIO should be mapped as device memory.

One more caveat for FWB, is guest DMA, which we assume to be
dma-coherent for now, otherwise we should just disable FWB for all
protected guests.

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