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 <[e-mail]>
Signed-off-by: Ralf Baechle <[e-mail]>

arch/mips/ar7/Makefile

···
 	platform.o \
 	gpio.o \
 	clock.o
+EXTRA_CFLAGS += -Werror