mmc: sdhci-iproc: add device tree bindings

Add device tree binding documentation for IPROC SDHCI driver.

Acked-by: Ray Jui <rjui@broadcom.com>
Signed-off-by: Corneliu Doban <cdoban@broadcom.com>
Signed-off-by: Scott Branden <sbranden@broadcom.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>