dt-bindings: net: phy: Document support for external PHY clk

Ethern PHYs can have external an clock that needs to be activated before
communicating with the PHY.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Detlev Casanova <detlev.casanova@collabora.com>
Signed-off-by: David S. Miller <davem@davemloft.net>