mmc: mmci: replace blksz_datactrlXX by get_datactrl_cfg callback

This patch allows to get datactrl configuration specific
at variant. This introduce more flexibility on datactlr
value.

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