dt: cpsw: phy-handle, phy_id, and fixed-link are mutually exclusive

The phy-handle, phy_id, and fixed-link properties are mutually exclusive,
and only one need be specified. Make this clear in the binding doc.

Also mark the phy_id property as deprecated, as phy-handle should be
used instead.

Signed-off-by: David Rivshin <drivshin@allworx.com>
Reviewed-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>