MIPS: AR7: Override CFLAGS with -Werror

Now that we have removed all warnings from the ar7 board code we can use
-Werror like on other MIPS boards.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
diff --git a/arch/mips/ar7/Makefile b/arch/mips/ar7/Makefile
index 7435e44..26bc5da 100644
--- a/arch/mips/ar7/Makefile
+++ b/arch/mips/ar7/Makefile
@@ -8,3 +8,4 @@
 	platform.o \
 	gpio.o \
 	clock.o
+EXTRA_CFLAGS += -Werror