target/armv7m: rework Cortex-M register handling part 2

target/armv7m: rework Cortex-M register handling part 2

Make arm register id coherent with reg_list index.
Without this reg_list[ARMV7M_R12] was possible but
reg_list[ARMV7M_FPSCR] was out of bounds.

Remove unused items from reg_list index.

Change-Id: I84d3b5c496fc1839d07a5b74cb1fd1c3d4ff8989
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/5862
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Christopher Head <chead@zaber.com>