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>