dt-bindings: net: rename BCM4908 Ethernet binding

Rob pointed out that a normal convention is "brcm,bcm4908-enet" so
update whole binding to match it.

Suggested-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>