net: mvpp2: make the phy optional

There is not necessarily a PHY between the GoP and the physical port.
However, the driver currently makes the "phy" property mandatory,
contrary to what is stated in the device tree bindings. This patch makes
the PHY optional, and aligns the PPv2 driver on its device tree
documentation. However if a PHY is provided, the GoP link interrupt
won't be used.

With this patch switches directly connected to the serdes lanes and SFP
ports on the Armada 8040-db and Armada 7040-db can be used if the link
interrupt is described in the device tree.

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Tested-by: Marcin Wojtas <mw@semihalf.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>