sdhci-pci: enable SD card interface on Merrifield

Intel Merrifield provides an interface to an external SD card.
Enable it here.

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>