spi: rspi: Add support for specifying CPHA/CPOL

Add support for specifying the SPI clock phase and polarity, based on the
SDK reference code.

Signed-off-by: Geert Uytterhoeven <geert+renesas@linux-m68k.org>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mark Brown <broonie@linaro.org>