mmc: sdhci-pci: enable runtime PM for Medfield SDIO

Runtime PM for SDIO is no longer enabled by default (see
5c7f0e08) so it must now
be enabled per platform, in this case Medfield uses it.

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