dt-bindings: net: ethernet-controller: document label property

"label" provides human readable name used on a box, board or schematic
to identify Ethernet port.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220209082820.2210753-1-o.rempel@pengutronix.de