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: NSergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: NDavid 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-rmii-en: use RMII
 - 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
@@ -22,6 +21,7 @@ Optional properties:
 - phy-handle: Contains a phandle to an Ethernet PHY.
               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
 
 Example (enbw_cmc board):
 	eth0: emac@1e20000 {