spi: omap2-mcspi: Convert to use GPIO descriptors

spi: omap2-mcspi: Convert to use GPIO descriptors

The OMAP2 MCSPI has some kind of half-baked GPIO CS support:
it includes code like this:

  if (gpio_is_valid(spi->cs_gpio)) {
        ret = gpio_request(spi->cs_gpio, dev_name(&spi->dev));
	(...)

But it doesn't parse the "cs-gpios" attribute in the device
tree to count the number of GPIOs or pick out the GPIO numbers
and put these in the SPI master's .cs_gpios property.

We complete the implementation of supporting CS GPIOs
from the device tree and switch it over to use the SPI core
for this.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Tony Lindgren <tony@atomide.com>
Link: https://lore.kernel.org/r/20200625231257.280615-1-linus.walleij@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>