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>