mpc85xx, mpc86xx: Fix gd->cpu pointer after relocation

The gd->cpu pointer is set to an address located in flash when the
probecpu() function is called while U-Boot is executing from flash.
This pointer needs to be updated to point to an address in RAM after
relocation has occurred otherwise Linux may not be able to boot due to
"fdt board" crashing if flash has been erased or changed.

This bug was introduced in commit
a0e2066f.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
Reported-by: Ed Swarthout <Ed.Swarthout@freescale.com>
Tested-by: Kumar Gala <galak@kernel.crashing.org>
Tested on MPC8527DS.
Tested by: Ed Swarthout <Ed.Swarthout@freescale.com>