staging: dwc2: update DT binding to add generic clock/phy properties

dwc2/s3c-hsotg require a single clock to be specified and optionally
a generic phy. On the s3c-hsotg driver old style USB phy support is
present as a fallback so the generic phy properties are optional.

Signed-off-by: Matt Porter <mporter@linaro.org>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>