mmc: sdhci-pci: add platform data

Add a means of getting platform data for the SDHCI PCI
devices.  The data is stored against the slot not the
device in order to support multi-slot devices.

The data allows platform-specific setup (such as getting
GPIO numbers from firmware or setting up wl12xx for SDIO)
to be done in platform support files instead of the
sdhci-pci driver.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Chris Ball <cjb@laptop.org>