mmc: sdhci-pci: 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_pci_fixes structure, then sdhci_pci_probe_slot() will allocate extra
space for it, and sdhci_pci_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>
Tested-by: Ludovic Desroches <ludovic.desroches@microchip.com>