mmc: mmci: add 8bit bus support in variant data

This patch adds 8bit bus enable to variant structure giving more flexibility
to the driver to support more SOCs which have different clock register layout.

Without this patch other new SOCs like Qcom will have to add more code
to special case them.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>