avr32: Reduce DataFlash bus speed to 8 MHz on ATNGW100

Doing this in combination with "atmel_spi: fix hang due to missed
interrupt" appears to eliminate the overruns I'm seeing when using
JFFS2-on-DataFlash as /usr filesystem on the ATNGW100.

Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>