jtag/drivers: Add dmem driver

jtag/drivers: Add dmem driver

Direct memory driver support for CoreSight Access Port(AP).

Even though we emulate SWD (serial wire debug), we aren't actually
using swd. Instead, we are using a direct memory access to get to the
register set. This is similar in approach to other fast access native
drivers such as am335xgpio drivers.

Example operation on Texas Instrument's AM62x K3 SoC:

+-----------+
|  OpenOCD  |   SoC mem map
|    on     |--------------+
| Cortex-A53|              |
+-----------+              |
                           |
+-----------+        +-----v-----+
|Cortex-M4F |<───────|           |
+-----------+        |           |
                     |  DebugSS  |
+-----------+        |           |
|Cortex-M4F |<───────|           |
+-----------+        +-----------+

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Jason Peck <jpeck@ti.com>
Change-Id: I8470cb15348863dd844b2c0e3f63a9063cb032c6
Reviewed-on: https://review.openocd.org/c/openocd/+/7088
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>