target/adi_v5_swd: add support for SWD multidrop

target/adi_v5_swd: add support for SWD multidrop

Based on Graham Sanderson's http://review.openocd.org/4935

Unlike Graham Sanderson's version this patch does not add
any multidrop specific queuing.

Multidrop SWD is handled mostly by the same code as single SWD,
just a selection sequence is prepended to a SWD operation
as needed.

This is a minimal working implementation without checking for
configuration errors (mixing multidrop and non multidrop DPs,
multiple use of the same selection id etc...).

Multidrop switching likely demands changes in the adapter code.

Change-Id: I99a5742c209b49c0483e800f6105cb5e59a897d9
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: https://review.openocd.org/c/openocd/+/6141
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>