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

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

This patch updates the binding information to reflect the
extra dt options which are now supported by the sdhci-st.c
driver which enable support for stih407 family silicon.

STiH410 SoC and later support UHS modes for eMMC, so the
driver now makes use of these common bindings. Examples
are provided for both eMMC (which has additional bindings)
and also sd slot for STiH407.

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Acked-by: Maxime Coquelin <maxime.coquelin@st.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>