tests/subsys/storage/flash_map: get flash device from flash area

Get flash device from flash area instead of the hardcoded
DT_CHOSEN_ZEPHYR_FLASH_CONTROLLER_LABEL. This makes the test
possible to be ran on device that has image_1 defined in
external flash.

Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>