dt-bindings: net: dsa: maxlinear,mxl862xx: remove port label

The ports in the example device tree should not have a 'label'
property. Labels for all user ports have been removed from an earlier
submission, but this was overlooked in the case of the CPU port.

Remove 'cpu' port label from the example.

Suggested-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/61579de297eb636ec5f1e6c97d453e26abb0625d.1772507210.git.daniel@makrotopia.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>