swd: get rid of jtag queue to assert/deassert srst

swd: get rid of jtag queue to assert/deassert srst

The transport SWD uses the JTAG queue to assert/deassert the
system reset srst. This is the major inconsistency that has to be
removed to properly split JTAG and SWD.

Introduce a new driver API, reset(), to controls both the signals
trst and srst in the driver, skipping the JTAG queue. Put the new
API in struct jtag_interface, even if in this patch it's used for
SWD only; the goal is to get it reused by the other transports.

Add the implementation of the API in all the drivers that
implement SWD. Such implementation is almost the same of the old
code in JTAG queue.

Create a wrapper adapter_system_reset() to use the new API and
remove the SWD specific swd_add_reset(). In the wrapper replace
jtag_add_sleep() with jtag_sleep(), because the former uses the
JTAG queue too.
Rename the old jtag_add_reset() as legacy_jtag_add_reset() with
the target to remove it when all drivers would be ported to the
new reset API. Create a new jtag_add_reset() that calls the
legacy function for drivers still on the old reset API.

Use the new API also on JTAG transport for the drivers that can
support both SWD and JTAG.

For the moment, do not modify the implementation of JTAG-only
drivers, which will continue using the usual method. This should
be cleaned-up in future commits.

Change-Id: I32331c88313f6059b25e12c6bb0156aebc1c074f
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4895
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>