cortex_m: use the new enum ARMV7M_REGSEL_name

cortex_m: use the new enum ARMV7M_REGSEL_name

Register xPSR is indexed directly with its value 16 or with the
incorrect enum ARMV7M_xPSR.

Replace them with the new enum ARMV7M_REGSEL_xPSR.

Change-Id: I86600e7f78e39002ce45f66d4792d5067c1f541b
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5873
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>