ppc: Revert patch 70431e8a that used _start instead of CFG_MONITOR_BASE

The patch 70431e8a (Make MPC83xx one step
closer to full relocation.) doesn't use CFG_MONITOR_BASE anymore. But
on 4xx systems _start currently cannot be used for this calculation.
So revert back to the original version for now.

Signed-off-by: Stefan Roese <sr@denx.de>