spi: rspi: SPI DMA core needs both RX and TX DMA to function

spi: rspi: SPI DMA core needs both RX and TX DMA to function

The SPI DMA core framework needs both RX and TX DMA to function.  As a
preparation for converting the driver to use this framework, fall back to
PIO if no DMA channel or only one DMA channel is available.

This affects only RSPI, which could do DMA transfers for TX-only before.
RSPI-RZ and QSPI (at least for Single SPI Transfers) will need both RX and
TX DMA anyway.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Mark Brown <broonie@linaro.org>
(cherry picked from commit 5f338d0ce0b4c9e6c554b92cfb288789a41bfbc1)
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>