of: add helper function to retrive match data

It's a common operation for device drivers to retrive the data
member from of_device_id struct in their probe function.

Most driver end up doing:
    const struct of_device_id *match;
    match = of_match_device(driver_of_match, &pdev->dev);
    driver->data = match->data;

With the of_device_get_match_data helper function all this can
done in one go.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
[robh: add missing inline to dummmy declaration]
Signed-off-by: Rob Herring <robh@kernel.org>