mmc: mmci: add variant property to define dpsm bit

This patch adds datactrl variant property to define
dpsm enable bit. Needed to support the STM32 variant
(STM32 has no dpsm enable bit).

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