doc: Add "ti,am437x-dwc3" comaptible for dwc3 glue

Add the compatible "ti,am437x-dwc3" for dwc3 glue driver.

Signed-off-by: George Cherian <george.cherian@ti.com>
Acked-by: Roger Quadros <rogerq@ti.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>