dt-bindings: net: fsl,gianfar-mdio: Update information about TBI

dt-bindings: net: fsl,gianfar-mdio: Update information about TBI

When this binding was originally written, all known TSEC Ethernet
controllers had a Ten-Bit Interface (TBI). However, some datasheets such
as for the MPC8315E suggest that this is not universally true:

  The eTSECs do not support TBI, GMII, and FIFO operating modes, so all
  references to these interfaces and features should be ignored for this
  device.

Acked-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: J. Neuschäfer <j.ne@posteo.net>
Link: https://patch.msgid.link/20250228-gianfar-yaml-v2-2-6beeefbd4818@posteo.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>