mips: replace deprecated EXTRA_CFLAGS with ccflags-y

As Documentation/kbuild/makefiles.rst says, EXTRA_CFLAGS is deprecated.
Replace it with ccflags-y.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>