mmc: slot-gpio: Delete legacy GPIO handling

All host drivers are converted to look up GPIO descriptors
from device tree, ACPI or machine descriptor tables, so now
we can delete the legacy GPIO handling using hardcoded GPIO
numbers from the kernel.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>