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: NStefan Roese <sr@denx.de>

lib_ppc/board.c

@@ -440,7 +440,7 @@ void board_init_f (ulong bootflag)
 	 *  - monitor code
 	 *  - board info struct
 	 */
-	len = (ulong)&_end - (ulong)&_start + EXC_OFF_SYS_RESET;
+	len = (ulong)&_end - CFG_MONITOR_BASE;
 
 	/*
 	 * Subtract specified amount of memory to hide so that it won't