target/armv7m: rework Cortex-M register handling part 1

target/armv7m: rework Cortex-M register handling part 1

Define a new enum with DCRSR.REGSEL selectors.
Introduce armv7m_map_id_to_regsel() to unify mapping in one place.
Use DCRSR.REGSEL selectors for low level register read/write.

Change-Id: Ida0ccdfa9cdb1257a1900b8bfbf172b076374d39
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/5327
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Christopher Head <chead@zaber.com>