mmc: mmci: add variant property to write datactrl before command

This patch adds a boolean property to allow to write datactrl
before to send command, whatever the command type (read or write).
Needed to support the STM32 sdmmc variant.

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