rtos: zephyr: add zephyr_params for cortex r4

rtos: zephyr: add zephyr_params for cortex r4

Implementation for Cortex-M does works for Cortex-R too,
it allows me to fetch thread list and their backtrace on
a Cortex-R platforms.

Change-Id: I23e6eb00879587ba36e0bfb560f7002a9653d39b
Signed-off-by: Julien Massot <julien.massot@iot.bzh>
Reviewed-on: http://openocd.zylin.com/6369
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>