spi: rspi: Add DMA support for QSPI on R-Car Gen2

spi: rspi: Add DMA support for QSPI on R-Car Gen2

Enable DMA support for QSPI on R-Car Gen2, for Single, Dual, and Quad SPI
Transfers.

Performance figures for reading from a QSPI FLASH driven at 24.375 MHz
on r8a7791/koelsch:
  - Single:  1.1 Mbps PIO, 23 Mbps DMA
  - Dual  : 12.7 Mbps PIO, 48 Mbps DMA
  - Quad  : 13   Mbps PIO, 70 Mbps DMA

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Mark Brown <broonie@linaro.org>
(cherry picked from commit 4f12b5e529e4ff274eb478ec1c2ef41358ed9577)
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>