dt-bindings: usb: ti,keystone-dwc3.yaml: Improve schema

There were some review comments after the patch was integrated.
Address those.

Fixes: 1883a934 ("dt-bindings: usb: convert keystone-usb.txt to YAML")
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>