dt: Add Add eth_max_speed override for Pi5, Pi500, and CM5.

max-speed is a generic property for ethernet PHYs, so is
supported by the PHY on Pi5, Pi500, and CM5.

Add the override and update the documentation accordingly.

Signed-off-by: Dave Stevenson <[email protected]>

Author: 6by9

arch/arm/boot/dts/overlays/README

@@ -266,8 +266,8 @@ Params:
 
         eth_max_speed           Set the maximum speed a link is allowed
                                 to negotiate. Legal values are 10, 100 and
-                                1000 (default set by the platform).
-                                Pi3B+, Pi4, Pi400, and CM4 only.
+                                1000 (default set by the platform). Pi3B+,
+                                Pi4, Pi400, CM4, Pi5, Pi500, and CM5 only.
 
         fan_temp0               Temperature threshold (in millicelcius) for
                                 1st cooling level (default 50000). Pi5 only.

arch/arm64/boot/dts/broadcom/bcm2712-rpi.dtsi

@@ -194,6 +194,7 @@ watchdog: &pm {};
 		drm_fb2_vc4 = <&aliases>, "drm-fb2=",&vc4;
 		eth_led0 = <&phy1>,"led-modes:0";
 		eth_led1 = <&phy1>,"led-modes:4";
+		eth_max_speed = <&phy1>,"max-speed:0";
 		fan_temp0 = <&cpu_tepid>,"temperature:0";
 		fan_temp0_hyst = <&cpu_tepid>,"hysteresis:0";
 		fan_temp0_speed = <&fan>, "cooling-levels:4";