spi: spi-fsl-dspi: XSPI FIFO handling (in TCFQ mode)

This implements handling of split CMD and TX FIFO queues for XSPI when
running in TCFQ mode.

It should be simple to add it to EOQ mode also.  Currently, EOQ mode is
only used with coldfire.  So if coldfire DSPI supports XSPI, XSPI FIFO
handling should be added to EOQ mode also.

Signed-off-by: Esben Haabendal <eha@deif.com>
Acked-by: Martin Hundebøll <martin@geanix.com>
Signed-off-by: Mark Brown <broonie@kernel.org>