mmc: mmci: add prepare/unprepare_data callbacks

This patch adds prepare/unprepare callbacks to mmci_host_ops.
Like this mmci_pre/post_request can be generic, mmci_prepare_data
and mmci_unprepare_data provide common next_cookie management.

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