[POWERPC] Use mpc8641hpcn PIC base address from dev tree.

After going through the trouble of setting up the PIC base
address in the pic@40000 device tree node, use it instead
of the obsolete hard-coded value.

Signed-off-by: Jon Loeliger <jdl@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>