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>

Documentation/devicetree/bindings/net/davinci_emac.txt

@@ -10,7 +10,6 @@ Required properties:
 - ti,davinci-ctrl-mod-reg-offset: offset to control module register
 - ti,davinci-ctrl-ram-offset: offset to control module ram
 - ti,davinci-ctrl-ram-size: size of control module ram
-- ti,davinci-no-bd-ram: has the emac controller BD RAM
 - interrupts: interrupt mapping for the davinci emac interrupts sources:
               4 sources: <Receive Threshold Interrupt
 			  Receive Interrupt
@@ -22,6 +21,7 @@ Optional properties:
               If absent, davinci_emac driver defaults to 100/FULL.
 - local-mac-address : 6 bytes, mac address
 - ti,davinci-rmii-en: 1 byte, 1 means use RMII
+- ti,davinci-no-bd-ram: boolean, does EMAC have BD RAM?
 
 Example (enbw_cmc board):
 	eth0: emac@1e20000 {