dmaengine: at_hdmac: add FIFO configuration parameter to DMA DT binding

For most devices the FIFO configuration is the same i.e. when half FIFO size is
available/filled, a source/destination request is serviced. But USART devices
have to do it when there is enough space/data available to perform a single
AHB access so the ASAP configuration.

Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>