mmc: sdhci: sirf: 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.

If the retrieveal is successful, we also request the IRQ.

As a result the private subdriver data can be removed
entirely.

Cc: Weijun Yang <york.yang@csr.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>