Something seems to be missing in regards to flushing specific ranges of
the TLB.  For the moment, flushing the entire thing seems to make it
work alright.

Should give 39-bit DMA addressing on the relevant chipsets.

v2: allocate contig 16KiB for dummy pages, reported by mwk on irc
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>