mmc: sdhci-msm: Qualcomm SDHCI binding documentation

This patch adds the device-tree binding documentation for Qualcomm
SDHCI driver. It contains the differences between the core properties
in mmc.txt and the properties used by the sdhci-msm driver.

Signed-off-by: Georgi Djakov <gdjakov@mm-sol.com>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <chris@printf.net>