arm: nordic: flash: Use FLASH_DEV_NAME define

We want to move to use a common FLASH_DEV_NAME across the various flash
drivers. So samples, tests, or other code can be a bit more generic.  So
replace CONFIG_SOC_FLASH_NRF5_DEV_NAME with FLASH_DEV_NAME.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>