spi: rspi: Use core SPI_MASTER_MUST_[RT]X handling

RSPI needs dummy transfers to generate the SPI clock on receive.
RSPI-RZ and QSPI always do both transmit and receive.

Use the SPI core SPI_MASTER_MUST_RX/SPI_MASTER_MUST_TX infrastructure
instead of checking for the presence of buffers and providing dummy data
ourselves (for PIO), or providing a dummy buffer (for DMA).

rspi_receive_dma() now provides full duplex DMA transfers on RSPI, and is
renamed to rspi_send_receive_dma().

As the SPI core will always provide a TX buffer, the logic to choose
between DMA send and DMA send/receive in rspi_transfer_one() now has to
check for the presence of an RX buffer. Likewise for the DMA availability
tests in rspi_is_dma().

The buffer tests in qspi_transfer_one() are now always true, so they're
removed.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Mark Brown <broonie@linaro.org>