tty: n_smux: Add support for SMUX subsystem restart

Support subsystem restart for SMUX which includes being able to load and
unload the SMUX TTY Line Discipline.

The TTY port is shared between SMUX and memory dump collection which
requires support for gracefully disconnecting after a remote modem
crash and then reconnect after the modem has been rebooted.

CRs-Fixed: 368263
Change-Id: I0fea4c5f0f13c88282f33a7677ac5fd16dc690d9
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>
Signed-off-by: David Brown <davidb@codeaurora.org>