dt: bindings: Make compatible optional for mmc function nodes

On some boards (android tablets) different batches use different sdio
wifi modules. This is not a problem since mmc/sdio is an enumerable bus,
so we only need to describe and activate the mmc controller in dt and
then the kernel will automatically load the right driver.

Sometimes it is useful to specify certain ethernet properties for these
"unknown" sdio devices, specifically we want the boot-loader to be able
to set "local-mac-address" as some of these sdio wifi modules come without
an eeprom / without a factory programmed mac address.

Since the exact device is unknown (differs per batch) we cannot use
a wifi-chip specific compatible, thus sometimes it is desirable to have a
mmc function node, without having to make up an otherwise unused compatible
for the node, so make the compatible property optional.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>