mmc: sdhci-iproc: Add brcm, sdhci-iproc compat string in bindings document

Adds brcm,sdhci-iproc compat string to DT bindings document for
the iProc SDHCI driver.

Signed-off-by: Anup Patel <anup.patel@broadcom.com>
Signed-off-by: Scott Branden <scott.branden@broadcom.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>