spi: rspi: Add support for multiple native chip selects

RSPI variants on some SuperH or R-Mobile SoCs support multiple native
chip selects. Add support for this by configuring the SSL Assert Signal
Setting.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200102133822.29346-6-geert+renesas@glider.be


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