UPSTREAM: mmc: Add SDIO function devicetree subnode parsing

(cherry-picked from 25185f3f31c924eecc6ff4f55f7acceabf24de11)

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.

Change-Id: I40b79a5d05bc4d5f66f24099f1ec87c8c7b2b228
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>