zephyr: added ISB after cortex control was set

After modifying the CONTROL register, ISB function should be
used to ensure the effect of the change applies to
subsequent code.
Due to the simple nature of the Cortex-M3, Cortex-M4, Cortex-M0+,
Cortex-M0, and Cortex-M1 pipeline,
omission of the ISB instruction does not cause any problem.
But might cause problem on a more advanced core.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>