spi: pic32: Retire dma_request_slave_channel_compat()

There is no reason to use the dma_request_slave_channel_compat() as no
filter function and parameter is provided.

Switch the driver to use dma_request_chan() instead and add support for
deferred probing against DMA channel.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Reviewed-by: Vinod Koul <vkoul@kernel.org>
Link: https://lore.kernel.org/r/20191121092703.30465-1-peter.ujfalusi@ti.com
Signed-off-by: Mark Brown <broonie@kernel.org>