powerpc: Fix xmon for systems without MSR[RI]

Based on patch by David Gibson <dwg@au1.ibm.com>

xmon has a longstanding bug on systems which are SMP-capable but lack
the MSR[RI] bit.  In these cases, xmon invoked by IPI on secondary
CPUs will not properly keep quiet, but will print stuff, thereby
garbling the primary xmon's output.  This patch fixes it, by ignoring
the RI bit if the processor does not support it.

There's already a version of this for 4xx upstream, which we'll need
to extend to other RI-lacking CPUs at some point.  For now this adds
Book3e processors to the mix.

Signed-off-by: Jimi Xenidis <jimix@pobox.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>