spi: lantiq-ssc: Convert to use GPIO descriptors

spi: lantiq-ssc: Convert to use GPIO descriptors

This switches the Lantiq SSC driver over to use GPIO descriptor
handling in the core.

The driver was already utilizing the core to look up and request
GPIOs from the device tree so this is a pretty small change
just switching it over to use descriptors directly instead.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Link: https://lore.kernel.org/r/20200625202149.209276-1-linus.walleij@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>