boards: nrf9160dk_nrf9160: Apply a few corrections to revision 0.14.0

- remove deprecated `label` properties
- add `spi-flash0` alias
- correct `sfdp-bfp` property value to reflect exactly what is returned
  by the flash chip

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>