of: provide a binding for the 'fixed-link' property

Some Ethernet MACs have a "fixed link", and are not connected to a
normal MDIO-managed PHY device. For those situations, a Device Tree
binding allows to describe a "fixed link", as a "fixed-link" property
of the Ethernet device Device Tree node.

This patch adds:

 * A documentation for the Device Tree property "fixed-link".

 * A of_phy_register_fixed_link() OF helper, which provided an OF node
   that contains a "fixed-link" property, registers the corresponding
   fixed PHY.

 * Removes the warning on the of_phy_connect_fixed_link() that says
   new drivers should not use it, since Grant Likely indicated that
   this "fixed-link" property is indeed the way to go.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
net: phy: call mdiobus_scan() after adding a fixed PHY

The fixed_phy_add() function allows to register a fixed PHY. However,
when this function gets called *after* fixed_mdio_bus_init() (which
gets called at the module_init stage), then the fixed PHY is not
registered into the phylib.

In order to address this, we add a call to mdiobus_scan() in
fixed_phy_add() to ensure that the PHY indeed gets registered into the
phylib, even if the fixed_phy_add() is called after
fixed_mdio_bus_init().

This is needed because until now, the only code that was calling the
fixed_add_phy() function was PowerPC-specific platform code, which
could ensure that such fixed PHYs get registered before
fixed_mdio_bus_init() is called.

However, with the new of_phy_register_fixed_link() function, device
drivers can parse their 'fixed-link' property and register a fixed PHY
at ->probe() time, which may happen after fixed_mdio_bus_init() is
called.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
net: mvneta: add support for fixed links

Following the introduction of of_phy_register_fixed_link(), this patch
introduces fixed link support in the mvneta driver, for Marvell Armada
370/XP SOCs.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>