spi: oc-tiny: Use GPIO descriptors

spi: oc-tiny: Use GPIO descriptors

Switch the OC Tiny driver over to handling CS GPIOs using
GPIO descriptors in the core.

This driver is entirely relying on GPIOs to be used for
chipselect, so let the core pick these out using either
device tree or machine descriptors.

There are no in-tree users of this driver so no board files
need to be patched, out-of-tree boardfiles can use machine
descriptor tables, c.f. commit 1dfbf334f123.

Cc: Thomas Chou <thomas@wytron.com.tw>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20191205092411.64341-1-linus.walleij@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>