spi: stm32: Switch to use GPIO descriptors for CS

This switches the STM32 SPI driver over to using GPIO
descriptors for chip select. Instead of the callbacks for
picking the GPIO lines using the legacy API we just let
the core handle it all using descriptors.

Cc: Fabien Dessenne <fabien.dessenne@st.com>
Cc: Amelie Delaunay <amelie.delaunay@st.com>
Cc: Cezary Gapinski <cezary.gapinski@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20191205083401.27077-1-linus.walleij@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>