[PARISC] PA7200 also supports prefetch for read

It seems PA7200 processors also suppress traps on loads to
%r0. This means we can prefetch for read on these cpus. Of course,
we can't support prefetch for write, since that requires
LOAD DOUBLEWORD which was added with PA2.0

Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>