spi: rspi: Factor out handling of common mode bits

Basic SPI features like clock phase/polarity and loopback mode are
common to all RSPI variants.  Factor them out to reduce duplication.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200218105810.902-2-geert+renesas@glider.be


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