target/armv4_5: use c99 array designator to init arm_core_regs[]

target/armv4_5: use c99 array designator to init arm_core_regs[]

During code analysis and development, counting again and again the
lines to find the index of the register is a boring error-prone
brain-damaging activity.
Use the c99 syntax and add once forever the array designators to
specify the index values.
The code behavior is not changed.

Change-Id: I2c70f70794475679efb91a8dfadc00f50715bd3f
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5256
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>