DT: net: davinci_emac: "ti, davinci-no-bd-ram" property is actually optional

The "ti,davinci-no-bd-ram" property for the DaVinci EMAC binding simply can't be
required one, as it's boolean (which means it's absent if false).

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>