ppc: Set CFG_MEM_TOP_HIDE to 0 if not already defined

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

lib_ppc/board.c

@@ -120,6 +120,10 @@ DECLARE_GLOBAL_DATA_PTR;
 #define	TOTAL_MALLOC_LEN	CFG_MALLOC_LEN
 #endif
 
+#if !defined(CFG_MEM_TOP_HIDE)
+#define CFG_MEM_TOP_HIDE	0
+#endif
+
 extern ulong __init_end;
 extern ulong _end;
 ulong monitor_flash_len;