dt-bindings: phy: Add DT bindings documentation for NS2 USB DRD PHY

This patch adds DT bindings documentation for NS2 DRD PHY driver.

Signed-off-by: Raviteja Garimella <raviteja.garimella@broadcom.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>