usb: phy: tegra: Register as an USB PHY.

Register the Tegra PHY device instances with the PHY subsystem so that
the Tegra EHCI driver can locate a PHY via the standard APIs.

Signed-off-by: Tuomas Tynkkynen <ttynkkynen@nvidia.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>