[ppc] Fix build breakage for all non-4xx PowerPC variants.

- adapt to the more generic EXCEPTION_PROLOG and CRIT_EXCEPTION macros
- minor 4xx cleanup
diff --git a/include/mpc5xxx.h b/include/mpc5xxx.h
index 089aa13..a4581a3 100644
--- a/include/mpc5xxx.h
+++ b/include/mpc5xxx.h
@@ -39,6 +39,7 @@
 
 /* Exception offsets (PowerPC standard) */
 #define EXC_OFF_SYS_RESET	0x0100
+#define _START_OFFSET		EXC_OFF_SYS_RESET
 
 /* useful macros for manipulating CSx_START/STOP */
 #if defined(CONFIG_MGT5100)