mmc: Add SDIO function subnode DT documentation

While SDIO devices are runtime probable they sometimes need nonprobable
additional information on embedded systems, like an additional gpio
interrupt or a clock. This binding describes how to add child nodes to the
devicetree to supply this information.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
[hdegoede@redhat.com: Documented the need for #address- and #size-cells]
[hdegoede@redhat.com: Added a real world example]
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>