csky: Fixup ftrace modify panic

During ftrace init, linux will replace all function prologues
(call_mcout) with nops, but it need flush_dcache and
invalidate_icache to make it work. So flush_cache functions
couldn't be nested called by ftrace framework.
Signed-off-by: Guo Ren <guoren@linux.alibaba.com>

arch/csky/mm/Makefile

 # SPDX-License-Identifier: GPL-2.0-only
 ifeq ($(CONFIG_CPU_HAS_CACHEV2),y)
 obj-y +=			cachev2.o
+CFLAGS_REMOVE_cachev2.o = $(CC_FLAGS_FTRACE)
 else
 obj-y +=			cachev1.o
+CFLAGS_REMOVE_cachev1.o = $(CC_FLAGS_FTRACE)
 endif
 
 obj-y +=			dma-mapping.o