Replace DT_FLASH_DEV_NAME with DT_CHOSEN_ZEPHYR_FLASH_CONTROLLER_LABEL.
We now set zephyr,flash-controller in the chosen node of the device
tree to the flash controller device.
NOTE: For a SoCs with on die flash, this points to the controller and
not the 'soc-nv-flash' node. Typically the controller is the
parent of the 'soc-nv-flash' node).
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>