The USB PHY inside the AM335x device provides the specified transmission line termination impedance, which you are calling damping resistors.
All analog parameters related to the USB PHY, including the termination impedance, are trimmed during final test of the device and should be within the values specified by the USB specification. Therefore, PHY related register values do not need to be adjusted by the user. This is the reason we do not publish the details of these registers.
Regards,
Paul