spi: omap2-mcspi: convert to dma_request_slave_channel_compat()

spi: omap2-mcspi: convert to dma_request_slave_channel_compat()

Convert dmaengine channel requests to use
dma_request_slave_channel_compat(). This supports the DT case of
platforms requiring channel selection from either the OMAP DMA or
the EDMA engine. AM33xx only boots from DT and is the only user
implementing EDMA so in the !DT case we can default to the OMAP DMA
filter.

Signed-off-by: Matt Porter <mporter@ti.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>