[POWERPC] enable hardware watchpoints on cell blades

Ulrich Weigand has found that the hardware watchpoints on cell were not
working back in November :

http://ozlabs.org/pipermail/linuxppc-dev/2007-November/046135.html

This patch sets them during initialization.

Signed-off-by: Jens Osterkamp <jens@de.ibm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>