usb: phy: tegra: Use DT helpers for dr_mode

Use the new of_usb_get_dr_mode helper function for parsing dr_mode
from the device tree. Also replace the usage of the custom
tegra_usb_phy_mode enum with the standard enum.

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>