mmc: sdhci-pci: add Intel Merrifield support

Implement initial SDHCI Intel Merrifield support.  This patch is based
on previous one from Yunpeng Gao <yunpeng.gao@intel.com>.

Signed-off-by: David Cohen <david.a.cohen@linux.intel.com>
Signed-off-by: Chris Ball <cjb@laptop.org>