spi: mpc512x-psc: Convert to use GPIO descriptors

spi: mpc512x-psc: Convert to use GPIO descriptors

This driver is already relying on the core to provide
valid GPIO numbers in spi->cs_gpio through
of_spi_get_gpio_numbers(), so we can switch to letting
the core use GPIO descriptors instead.

The driver was assigning a local function to the custom
chipselect callback, but I chose to just open code the
gpiod setting instead, this is easier to read.

The only platform that overrides the cs_control callback
is the mpc832x_rdb.

Cc: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Cc: Anatolij Gustschin <agust@denx.de>
Cc: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20220120002600.216667-1-linus.walleij@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>