dt-bindings: usb: dwc2: add disable-over-current

The driver parses disable-over-current protection and some
implementations use it (e.g. Altera Stratix10), so document it in the
bindings.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>