mmc: mmci: Add STM32 variant

STM32F4 and STM32F7 MCUs has a SDIO controller that looks like
an ARM PL810.
This patch adds the STM32 variant so that mmci driver supports it.

Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>