As a first step to improve the variant specific code for mmci, add a
->dma_setup() callback to the struct mmci_host_ops.

To show its use, let's deploy the callback for the qcom dml, which involves
also to the assign the mmci_host_ops pointer from the variant ->init()
callback.
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Ludovic Barre <ludovic.barre@st.com>