tty: bfin_jtag_comm: emulate a TTY over the Blackfin EMUDAT/JTAG interface

The Blackfin JTAG interface has a 4 byte generic data field (EMUDAT).  With
a little creative thinking, we can turn this into a TTY device.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Bryan Wu <cooloney@kernel.org>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>