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: NFlorian Fainelli <florian@openwrt.org>
Signed-off-by: NRalf Baechle <ralf@linux-mips.org>

arch/mips/ar7/Makefile

@@ -8,3 +8,4 @@ obj-y := \
 	platform.o \
 	gpio.o \
 	clock.o
+EXTRA_CFLAGS += -Werror