mem_ap: fix target arch_info type

The target mem_ap appears as an ARM target, thus it allows the
execution of ARM specific commands causing the crash of OpenOCD.
E.g. 'arm mrc ...' can be executed and segfaults.

Replace the incorrect ARM magic number with a dedicated one.
While there, remove the 'struct arm', that is now holding only the
mem_ap's dap, and replace it with a pointer to the dap.

Change-Id: I881332d3fdf8d8f8271b8711607737b052a5699b
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/6213
Tested-by: jenkins