dt-bindings: net: cdns,macb: Add compatible for Raspberry Pi RP1

The Raspberry Pi RP1 chip has the Cadence GEM ethernet
controller, so add a compatible string for it.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Stanimir Varbanov <svarbanov@suse.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea@tuxon.dev>
Link: https://patch.msgid.link/20250822093440.53941-3-svarbanov@suse.de


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