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

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

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>