i2c: at91: convert to dma_request_slave_channel_compat()

Use generic DMA DT helper. Platforms booting with or without DT populated are
both supported.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>