dt-bindings: net: dp83822: add constraints for mac-termination-ohms

Property mac-termination-ohms is defined in ethernet-phy.yaml. Add allowed
values for the property.

Signed-off-by: Dimitri Fedrau <dimitri.fedrau@liebherr.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/20250416-dp83822-mac-impedance-v3-2-028ac426cddb@liebherr.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>