net: dsa: mxl-gsw1xx: Support R(G)MII slew rate configuration

net: dsa: mxl-gsw1xx: Support R(G)MII slew rate configuration

Support newly introduced maxlinear,slew-rate-txc and
maxlinear,slew-rate-txd device tree properties to configure R(G)MII
interface pins' slew rate. It might be used to reduce the radiated
emissions.

Reviewed-by: Daniel Golle <daniel@makrotopia.org>
Tested-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@siemens.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://patch.msgid.link/20260114104509.618984-3-alexander.sverdlin@siemens.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>