mmc: mmci: add variant property to not read datacnt

This patch adds a boolean property to not read datacnt register.
Needed to support the STM32 sdmmc variant. MMCIDATACNT
register should be read only after the data transfer is completed.
When reading after an error event the read data count value may be
different from the real number of data bytes transferred.

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