target/mem_ap: generic mem-ap target

target/mem_ap: generic mem-ap target

This pseudo target allows attaching to any access point on the DAP at the
MEM-AP level and read and write addresses on the connected bus. For
example, one can create a mem_ap target on the APB-AP and read and write
registers of debug components directly. This allows many diagnostic
and other features be programmed entirely using TCL, without necessity
of adding drivers to OpenOCD.

Change-Id: I53229ffd68fb0f96fb68be15b0f3a76cc8843c8e
Signed-off-by: Matthias Welwarsky <matthias.welwarsky@sysgo.com>
Reviewed-on: http://openocd.zylin.com/4002
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Leonard Crestez <cdleonard@gmail.com>