drivers/vdebug: add support for DAP level interface

This patch adds support for DAP interface to Cadence vdebug driver.
It implements a new transport layer for dapdirect_swd.

Change-Id: I64b02a9e1ce91e552e07fca692879655496f88b6
Signed-off-by: Jacek Wuwer <jacekmw8@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6965
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>

tcl/target/vd_riscv.cfg

@@ -14,5 +14,4 @@ target create $_TARGETNAME riscv -chain-position $_TARGETNAME -coreid $_HARTID
 
 riscv set_reset_timeout_sec 120
 riscv set_command_timeout_sec 120
-# prefer to use sba for system bus access
-riscv set_prefer_sba on
+riscv set_mem_access sysbus progbuf