Documentation: broadcom-bcmgenet: add better clocks documentation

Document the required "clocks" phandles and their corresponding
"clock-names" properties for the two clocks used by the GENET hardware
block ("enet" and "enet-wol").

CC: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>