Fix the following coccicheck warning:

drivers/memory/tegra/tegra186-emc.c:158:0-23: WARNING:
  tegra186_emc_debug_max_rate_fops should be defined with DEFINE_DEBUGFS_ATTRIBUTE.
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Link: https://lore.kernel.org/r/1612684970-125948-1-git-send-email-jiapeng.chong@linux.alibaba.comSigned-off-by: Krzysztof Kozlowski <krzk@kernel.org>