bitbang: split jtag and swd operations

bitbang: split jtag and swd operations

The split in OpenOCD between SWD and JTAG has been already fully
implemented. The bitbang driver still keeps a single API write()
to drive the output lines.

Introduce a new SWD specific API swd_write().
Move the existing SWD bitbang drivers to the new API by extracting
the available conditional implementation.
Cleanup some function prototype.
Remove the now unused global swd_mode, handled implicitly.
Rename bitbang_exchange() as bitbang_swd_exchange() to track its
scope for SWD only.

Change-Id: Ie53080b941cb1ac7a34a1f80bad8bee4e304454d
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5555
Tested-by: jenkins