mmc: sdhci: add DDR50 1.8V mode support for BayTrail eMMC Controller

This is to enable DDR50 bus speed mode with 1.8V signaling capability
for BayTrail ACPI and PCI mode eMMC Controller.

Signed-off-by: Maurice Petallo <mauricex.r.petallo@intel.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>