powerpc/mpc512x: initialize board restart earlier

move the MPC512x restart initialization from the shared init routine
to the shared init_early routine

recent problems in the proc(5) filesystem initialization led to the
situation where the platform's restart routine was invoked yet the
registers required for software reset were not yet available, which
made the board hang instead of reboot

Signed-off-by: Gerhard Sittig <gsi@denx.de>
Signed-off-by: Anatolij Gustschin <agust@denx.de>