net: phy: add ofnode node to struct phy_device

Now the UCLASS_ETH device "node" field is owerwritten by some network drivers in
case of Ethernet PHYs which are linked to UCLASS_ETH device using
"phy-handle" DT property and when Ethernet PHY driver needs to read some
additional information from DT. In such cases following happens (in
general):

- network drivers
	priv->phydev = phy_connect(priv->bus, priv->phyaddr, dev,
				   priv->interface);
	<-- phydev is connected to dev which is UCLASS_ETH device

	if (priv->phy_of_handle > 0)
		dev_set_of_offset(priv->phydev->dev, priv->phy_of_handle);
	<-- phydev->dev->node is overwritten by phy-handle DT node

- PHY driver in .config() callback
	int node = dev_of_offset(dev);
	<-- PHY driver uses overwritten dev->node
        const void *fdt = gd->fdt_blob;

	 if (fdtdec_get_bool(fdt, node, "property"))
		...

As result, UCLASS_ETH device can't be used any more for DT accessing.

This patch adds additional ofnode node field to struct phy_device which can
be set explicitly by network drivers and used by PHY drivers, so
overwriting can be avoided. Also add helper function phy_get_ofnode()
which will check and return phy_device->node or dev_ofnode(phydev->dev) for
backward compatibility with existing drivers.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>