phy: tegra: xusb: Parse dual-role mode property

The device tree bindings document the "mode" property of "ports"
subnodes, but the driver was not parsing the property. In preparation
for adding role switching, parse the property at probe time.

Based on work by JC Kuo <jckuo@nvidia.com>.

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