powerpc: Add System RAM to /proc/iomem

We've resisted adding System RAM to /proc/iomem because it is
the wrong place for it. Unfortunately we continue to find tools
that rely on this behaviour so give up and add it in.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>