ppc4xx: Fix problem in 405EZ OCM initialization

As spotted by Bruce Adler this patch fixes an initialization problem
for the 405EZ OCM.

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

cpu/ppc4xx/start.S

@@ -783,7 +783,7 @@ _start:
 	mtdcr	ocmdscr2, r3            /* Set Data Side */
 	mtdcr	ocmiscr2, r3            /* Set Instruction Side */
 	addis	r3,0,0x0800             /* OCM Data Parity Disable - 1 Wait State */
-	mtdcr	ocmdsisdpc,r4
+	mtdcr	ocmdsisdpc,r3
 
 	isync
 #else /* CONFIG_405EZ */