mips: change to new flag variable

Replace EXTRA_CFLAGS with ccflags-y.

Signed-off-by: matt mooney <mfm@muteddisk.com>
Acked-by: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>

arch/mips/bcm63xx/boards/Makefile

@@ -1,3 +1,3 @@
 obj-$(CONFIG_BOARD_BCM963XX)		+= board_bcm963xx.o
 
-EXTRA_CFLAGS += -Werror
+ccflags-y := -Werror