tty: n_gsm line discipline

Add an implementation of GSM 0710 MUX. The implementation currently supports

- Basic and advanced framing (as either end of the link)
- UI or UIH data frames
- Adaption layer 1-4 (1 and 2 via tty, 3 and 4 as skbuff lists)
- Modem and control messages including the correct retry process
- Flow control

and exposes the MUX channels as a set of virtual tty devices including modem
signals. This is an experimental driver.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>