mmc: mmci: add variant property to set command stop bit

On cmd12 (STOP_TRANSMISSION), STM32 sdmmc variant needs to set
cmdstop bit in command register. The CPSM ("Command Path State Machine")
treats the command as a Stop Transmission command and signals
abort to the DPSM ("Data Path State Machine").

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