spi: Drop CONFIG_ATMEL_DATAFLASH_SPI

Last user of this option went away in commit:

fdc77189 ("board: usb_a9263: Update to support DT and DM")

Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
Acked-by: Wenyou Yang <wenyou.yang@microchip.com>