mmc: mmci: Add support to data commands via variant structure.

On some SOCs like Qcom there are explicit bits in the command register
to specify if its a data transfer command or not. So this patch adds
support to such bits in variant data, giving more flexibility to the
driver.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>