mmc: sdhci-pci: add 8-bit bus width support for mrst hc0

And hook platform_8bit_width to support 8-bit bus width.

Signed-off-by: Major Lee <major_lee@wistron.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Dirk Brandewie <dirk.brandewie@gmail.com>
Signed-off-by: Chris Ball <cjb@laptop.org>