mmc: sdhci-st: Intial support for ST SDHCI controller

This platform driver adds initial support for the SDHCI host controller
found on STMicroelectronics SoCs.

It has been tested on STiH41x b2020 platforms currently.

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Maxime Coquelin <maxime.coquelin@st.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>