cnss2: Add sysfs entry for system shutdown

Add sysfs entry '/sys/kernel/shutdown_wlan/shutdown'.
When system is shutting down or rebooting, this entry will be
written so that drvier can be notified and handle it properly.

Change-Id: I2537a2893f8787ff7c9d33608ef6c42075c3d128
Signed-off-by: Yu Wang <yyuwang@codeaurora.org>
Signed-off-by: Yue Ma <yuem@codeaurora.org>