[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/ppc4xx.h b/include/ppc4xx.h
index 8cead66..ca241d2 100644
--- a/include/ppc4xx.h
+++ b/include/ppc4xx.h
@@ -22,7 +22,8 @@
 #ifndef	__PPC4XX_H__
 #define __PPC4XX_H__
 
-#define _START_OFFSET 0x2100
+#define EXC_OFF_SYS_RESET	0x0100	/* System reset				*/
+#define _START_OFFSET		(EXC_OFF_SYS_RESET + 0x2000)
 
 #if defined(CONFIG_440)
 #include <ppc440.h>