spi: qup: support using GPIO as chip select line

Most of the device with QUP SPI adapter are actually using GPIO-s for
chip select.

However, this stopped working after ("spi: Retire legacy GPIO handling")
as it introduced a check on ->use_gpio_descriptors flag and since spi-qup
driver does not set the flag it meant that all of boards using GPIO-s and
with QUP adapter SPI devices stopped working.

So, to enable using GPIO-s again set ->use_gpio_descriptors to true and
populate ->max_native_cs.

Fixes: f48dc6b9

 ("spi: Retire legacy GPIO handling")
Signed-off-by: Robert Marko <robert.marko@sartura.hr>
Cc: luka.perkov@sartura.hr
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20221006194819.1536932-1-robert.marko@sartura.hr
Signed-off-by: Mark Brown <broonie@kernel.org>