dt-bindings: allow up to four clocks for orion-mdio

Armada 8040 needs four clocks to be enabled for MDIO accesses to work.
Update the binding to allow the extra clock to be specified.

Cc: stable@vger.kernel.org
Fixes: 6d6a331f

 ("dt-bindings: allow up to three clocks for orion-mdio")
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Josua Mayer <josua@solid-run.com>
Signed-off-by: David S. Miller <davem@davemloft.net>