mmc: sdhci-st: ST Microelectronics SDHCI binding documentation.

This patch adds the device tree binding documentation for the ST
SDHCI driver. It documents the differences between the core properties
described by mmc.txt and the properties used by the sdhci-st driver.

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>