DT: net: davinci_emac: "phy-handle" property is actually optional

Though described as required, the "phy-handle" property for the DaVinci EMAC
binding is actually optional, as the driver will happily function without it,
assuming 100/FULL link; the property is not specified  either in the example
device node,  or in the actual EMAC device nodes for DA850 and AM3517 device
trees.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>