DT: net: davinci_emac: "ti, davinci-rmii-en" property is actually optional

Though described as required, the "ti,davinci-rmii-en" property for the DaVinci
EMAC binding seems actually optional, as the driver should happily work without
it; the property is not specified either  in the example device node or in the
actual EMAC device node for DA850 device tree, only AM3517 one.

While at it, document the property better...

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