powerpc: mpc5121ads: update board config info in README

The config targets described in README are not present
any more, update the info to currently used targets.

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

board/freescale/mpc5121ads/README

@@ -1,7 +1,7 @@
 To configure for the current (Rev 3.x) ADS5121
-	make ads5121_config
+	make mpc5121ads_config
 This will automatically include PCI, the Real Time CLock, add backup flash
 ability and set the correct frequency and memory configuration.
 
 To configure for the older Rev 2 ADS5121 type (this will not have PCI)
-	make ads5121_rev2_config
+	make mpc5121ads_rev2_config