[POWERPC] PPC 4xx: Enable XMON on PPC 4xx boards

The following patch allows XMON to run on the 4xx platform.  Tested on
Walnut, Ebony, and Nova (440GX based) eval boards.  440EP, 440SP, and
440SPE boards should work as well.  Patch is against 2.6.18-rc6.

Signed-off-by: Josh Boyer <jdub@us.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>