mpc8xx/cpu.c: Fix GCC 4.6 build warnings

Fix:
cpu.c: In function 'check_CPU':
cpu.c:188:8: warning: variable 'mid' set but not used
[-Wunused-but-set-variable]

Signed-off-by: Wolfgang Denk <wd@denx.de>

arch/powerpc/cpu/mpc8xx/cpu.c

@@ -40,6 +40,7 @@
 #include <commproc.h>
 #include <netdev.h>
 #include <asm/cache.h>
+#include <linux/compiler.h>
 
 #if defined(CONFIG_OF_LIBFDT)
 #include <libfdt.h>
@@ -185,7 +186,7 @@ static int check_CPU (long clock, uint pvr, uint immr)
 	uint k, m;
 	char buf[32];
 	char pre = 'X';
-	char *mid = "xx";
+	__maybe_unused char *mid = "xx";
 	char *suf;
 
 	/* the highest 16 bits should be 0x0050 for a 8xx */