drivers: flash: nrf: Rename nrf5 to nrf

With upcoming ICs that are not in the nRF5x family, rename the flash
driver and all its dependencies from nrf5 to nrf.

Should also fix the issue introduced by f49150ca which broke the
assignment of the flash device due to a partial rename.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>