From: Benjamin Herrenschmidt <benh@kernel.crashing.org>

This patch adds those for ppc64, except for iSeries which cannot do
readq/writeq easily, at least not as far as I know but I need to ask the
iSeries specialists in Rochester to be sure.  But anyway, iSeries shouldn't
use anything in fb.h anyway ...

It also add the proper tweaks & barriers to make sure reads are actually
done right away and not delayed indefinitely (making the CPU think the read
data is actually used) and add necessary write barriers on IO writes.  For
some reasons, ppc64 in 2.6 lacked some of these, opening potential races
within some drivers.