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>

arch/mips/kvm/Makefile

@@ -4,7 +4,7 @@
 
 common-objs-y = $(addprefix ../../../virt/kvm/, kvm_main.o coalesced_mmio.o eventfd.o binary_stats.o)
 
-EXTRA_CFLAGS += -Ivirt/kvm -Iarch/mips/kvm
+ccflags-y += -Ivirt/kvm -Iarch/mips/kvm
 
 common-objs-$(CONFIG_CPU_HAS_MSA) += msa.o