spi: atmel: convert to dma_request_slave_channel_compat()

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

Based on Ludovic Desroches <ludovic.desroches@atmel.com> patchset
"ARM: at91: move to generic DMA device tree binding"

Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Acked-by: Ludovic Desroches <ludovic.desroches@gmail.com>
Signed-off-by: Mark Brown <broonie@linaro.org>