mmc: mmci: add get_datactrl_cfg callback and helper functions

This patch adds get_datactrl_cfg callback in mmci_host_ops
to allow to get datactrl configuration specific at variant.
Common helper function is defined and could be call by variant.

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