dt-bindings: eeprom: at25: Document device-specific compatible values

Document the recommended presence of a device-specific compatible value,
and list examples that are already in use or soon will be.
This will allow checkpatch to validate compatible values in DTS.

Update the example to match current best practices (generic node name,
specific compatible value first).
Signed-off-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: NRob Herring <robh@kernel.org>

Documentation/devicetree/bindings/eeprom/at25.txt

 EEPROMs (SPI) compatible with Atmel at25.
 
 Required properties:
-- compatible : "atmel,at25".
+- compatible : Should be "<vendor>,<type>", and generic value "atmel,at25".
+  Example "<vendor>,<type>" values:
+    "microchip,25lc040"
+    "st,m95m02"
+    "st,m95256"
+
 - reg : chip select number
 - spi-max-frequency : max spi frequency to use
 - pagesize : size of the eeprom page
@@ -22,8 +27,8 @@ Obsolete legacy properties can be used in place of "size", "pagesize",
 Additional compatible properties are also allowed.
 
 Example:
-	at25@0 {
-		compatible = "atmel,at25", "st,m95256";
+	eeprom@0 {
+		compatible = "st,m95256", "atmel,at25";
 		reg = <0>
 		spi-max-frequency = <5000000>;
 		spi-cpha;