dma: atmel-dma: correct typo in the device tree bindings documentation

The value to set ASAP mode for FIFO configuration is 2 instead of 1.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>