mmc: Add SDIO function devicetree subnode parsing

This adds SDIO devicetree subnode parsing to the mmc core. While
SDIO devices are runtime probable they sometimes need nonprobable
additional information on embedded systems, like an additional gpio
interrupt or a clock. This patch makes it possible to supply this
information from the devicetree. SDIO drivers will find a pointer
to the devicenode in their devices of_node pointer.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
[hdegoede@redhat.com: Misc. cleanups]
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>