sdhci-pci: enable SDIO interface on Intel Merrifield

Intel Merrifield is known to have an SDIO interface and on Intel Edison board a
WiFi card is wired to it.

Enable the interface here to allow WiFi card enumeration.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>