The STLB and the first command buffer (which is used to set up the TLBs)
has a 32 bit size restriction in hardware. There seems to be no way to
specify addresses larger than 32 bit. Keep it simple and restict the
addresses to the lower 4 GiB range for all coherent DMA memory
allocations.
Signed-off-by: Michael Walle <michael@xxxxxxxx>