spi: rspi: Add support for Quad and Dual SPI Transfers on QSPI

Add support for Quad and Dual SPI Transfers on the Renesas Quad Serial
Peripheral Interface, as found in R-Car Gen2 SoCs like R-Car H2 (r8a7790)
and R-Car M2 (r8a7791):
  - Add unidirectional transfer methods for Quad/Dual SPI Transfers.
  - Program the sequencer to handle SPI messages with multiple transfer
    modes when Quad or Dual transfers are enabled for an SPI slave.
    Up to 4 transfer modes per SPI message are supported by the hardware.
  - Advertise the availability of Quad and Dual SPI modes on QSPI.

Signed-off-by: Geert Uytterhoeven <geert+renesas@linux-m68k.org>
Signed-off-by: Mark Brown <broonie@linaro.org>