zephyr: Change DT_FLASH_AREA_ macros to FLASH_AREA_

As we replace the legacy DT_FLASH_ support use the new FLASH_AREA_
macros.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>
Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>