spi: ppc4xx: Convert to use GPIO descriptors

spi: ppc4xx: Convert to use GPIO descriptors

This converts the PPC4xx SPI driver to use GPIO descriptors.

The driver is already just picking some GPIOs from the device
tree so the conversion is pretty straight forward. However
this driver is looking form a pure "gpios" property rather
than the standard binding "cs-gpios" so we need to add a new
exception to the gpiolib OF parser to allow this for this
driver's compatibles.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Cc: linuxppc-dev@lists.ozlabs.org
Link: https://lore.kernel.org/r/20200714072226.26071-1-linus.walleij@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>