dt-bindings: Add Cygnus PCIe PHY binding doc

Add DT binding document for Broadcom Cygnus PCIe PHYs

Signed-off-by: Ray Jui <rjui@broadcom.com>
Reviewed-by: Arun Parameswaran <aparames@broadcom.com>
Reviewed-by: JD (Jiandong) Zheng <jdzheng@broadcom.com>
Reviewed-by: Scott Branden <sbranden@broadcom.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>