csky: Fixup ftrace modify panic

[ Upstream commit 359ae00d ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>