mmc: sdhci-acpi: Let devices define their own private data

Let devices define their own private data to facilitate device-specific
operations. The size of the private structure is specified in the
sdhci_acpi_slot structure, then sdhci_acpi_probe() will allocate extra
space for it, and sdhci_acpi_priv() can be used to get a reference to it.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>