ARM: tegra: Add compatible string for built-in ASIX on Colibri boards

The device tree node for the built-in ASIX Ethernet device on Colibri
boards needs a compatible string in order to pass DT schema validation.
Add the USB VID,PID compatible string as required by the DT schema for
USB devices.

Reviewed-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Tested-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>