spi: rspi: Add support for GPIO chip selects

Add support for GPIO chip selects using GPIO descriptors.  As the RSPI
controller always drives a native chip select when performing a
transfer, at least one native chip select must be left unused.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200102133822.29346-7-geert+renesas@glider.be


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