spi: npcm-pspi: Convert to use GPIO descriptors

The Nuvoton PSPI driver already uses the core to handle GPIO
chip selects but is using the old GPIO number method and
retrieveing the GPIOs in the probe() call.

Switch it over to using GPIO descriptors saving a bunch of
code and modernizing it.

Compile tested med ARMv7 multiplatform config augmented
with the Nuvoton arch and this driver.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Cc: Tomer Maimon <tmaimon77@gmail.com>
Link: https://lore.kernel.org/r/20200625225759.273911-1-linus.walleij@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>