phy: tegra: xusb: Add support to get companion USB 3 port

Tegra XUSB host, device mode driver requires the USB 3 companion port
number for corresponding USB 2 port. Add API to retrieve the same.

Signed-off-by: Nagarjuna Kristam <nkristam@nvidia.com>
Reviewed-by: JC Kuo <jckuo@nvidia.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>