dt-bindings: phy: Fix description of Tegra210 PHY nodes

The DT bindings document for the Tegra XUSB pad controller states that
'utmi' is one of the valid choices for the Tegra210 PHY node names and
has child lanes named, 'utmi-0', 'utmi-1', 'utmi-2' and 'utmi-3'.
However, neither the XUSB pad controller PHY driver or the actual
Tegra210 bindings for the XUSB pad controller use these names. Instead
both the driver and binding use the node name 'usb2' and for the child
lanes use the names 'usb2-0', 'usb2-1', 'usb2-2' and 'usb2-3'. Given
that the driver and binding are consistent with the naming, update the
DT bindings documentation to match.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>