spi: rspi: Fill in controller speed limits

Fill in the controller speed limits, so the SPI core can use them for
validating SPI transfers, and adjusting them where needed.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200819125904.20938-8-geert+renesas@glider.be


Signed-off-by: Mark Brown <broonie@kernel.org>