arm: cmsis: Convert DO_REBOOT to use CMSIS

CMSIS provides a complete implentation for reboot, we can utilize it
directly and reduce zephyr specific code.

Jira: ZEP-1568

Change-Id: Ia9d1abd5c1e02e724423b94867ea452bc806ef79
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>