spi: rspi: Add support for active-high chip selects

All RSPI variants support setting the polarity of the SSL signal.
Advertize support for active-high chip selects, and configure polarity
according to the state of the flag.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200309171537.21551-1-geert+renesas@glider.be


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