mmc: sdhci: spear: Use the slot GPIO descriptor

This driver is complicating things for no reason: the "cd"
GPIO can easily be retrieved from the device tree if present
using just mmc_gpiod_request_cd(), which will fetch the
descriptor from the device tree using the standard binding
just fine.

Cc: Viresh Kumar <vireshk@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>