dt-bindings: mmc: sprd: Add pinctrl support

When changing SD card voltage signal for Spreadtrum SD host controller,
it also need to switch related pin's state. Thus add pinctrl properties'
description in documentation.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>