remoteproc: pru: Replace DEFINE_SIMPLE_ATTRIBUTE with DEFINE_DEBUGFS_ATTRIBUTE

Fix the following coccicheck warning:
./drivers/remoteproc/pru_rproc.c:247:0-23: WARNING:
pru_rproc_debug_ss_fops should be defined with DEFINE_DEBUGFS_ATTRIBUTE

Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Link: https://lore.kernel.org/r/1614154829-42461-1-git-send-email-yang.lee@linux.alibaba.com


Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>