dt-bindings: net: dsa: lantiq,gswip: add MaxLinear R(G)MII slew rate

dt-bindings: net: dsa: lantiq,gswip: add MaxLinear R(G)MII slew rate

Add new maxlinear,slew-rate-txc and maxlinear,slew-rate-txd uint32
properties. The properties are only applicable for ports in R(G)MII mode
and allow for slew rate reduction in comparison to "normal" default
configuration with the purpose to reduce radiated emissions.

Signed-off-by: Alexander Sverdlin <alexander.sverdlin@siemens.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/20260114104509.618984-2-alexander.sverdlin@siemens.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>