devicetree: deprecate DT_CHOSEN_ZEPHYR_FLASH_CONTROLLER_LABEL

DT_CHOSEN_ZEPHYR_FLASH_CONTROLLER_LABEL was the last macro to be
deprecated from the DT_CHOSEN_()_LABEL series. All its usages have been
replaced with DT_CHOSEN(zephyr_flash_controller), usually combined with
DEVICE_DT_GET to obtain references at compile time.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>