Documentation: broadcom-bcmgenet: fix address and cells properties

This patch fixes a typo in the Device Tree binding for the
leading '#'.

Reported-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>