n_smux: add debugfs node for printing channel states

Add debugfs node that prints detailed state information
for all smux channels.

Change-Id: I727f1c53797ee317856056b8e4d9a8686940ce0f
Signed-off-by: Brent Hronik <bhronik@codeaurora.org>
Signed-off-by: Angshuman Sarkar <angshuman@codeaurora.org>
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>