arm: Use different enum for core_type and core_mode

arm: Use different enum for core_type and core_mode

The fields core_type and core_mode use the same enum arm_mode
but encode different information, making the code less immediate
to read.

Use a different enum arm_core_type for the field core_type.
The code behavior is not changed.

Change-Id: I60f2095ea6801dfe22f6da81ec295ca71ef90466
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5258
Tested-by: jenkins