mmc: mmci: add dma_start callback

This patch adds dma_start callback to mmci_host_ops.
Create a generic mmci_dma_start function which regroup
common action between variant.

Signed-off-by: Ludovic Barre <ludovic.barre@st.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>