mmc: mmci: introduce dma_priv pointer to mmci_host

-Introduces dma_priv pointer to define specific
needs for each dma engine. This patch is needed to prepare
sdmmc variant with internal dma which not use dmaengine API.
-Moves next cookie to mmci host structure to share same cookie
management between all variants.

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