usb: dwc3-generic: Allow different controller DT node pattern

usb: dwc3-generic: Allow different controller DT node pattern

The most of devicetree has the following USB node structure.
The controller node is placed as a child node of the glue node.
Current dwc3-generic driver works on this premise.

    glue {
        /* glue node */
	usb {
            /* controller node */
	};
    };

However, UniPhier original devicetree has the following USB node structure.
The controller node is separately placed from the glue node.

    usb {
        /* controller node */
    };
    glue {
        /* glue node */
    };

In dwc_glue_bind(), this patch provides .glue_get_ctrl_dev() callback to
get such a controller node and binds the driver related to the node.

If this callback isn't defined, dwc_glue_bind() looks for the controller
nodes from the child nodes, as before.

Suggested-by: Marek Vasut <marex@denx.de>
Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Reviewed-by: Marek Vasut <marex@denx.de>