phy: add Broadcom Kona USB2 PHY DT binding

Add a binding that describes the Broadcom Kona USB2 PHY found
on the BCM281xx family of SoCs.

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>