dt-bindings: rtc-2123: Make the node name generic

According to Devicetree Specification v0.2 document:

"The name of a node should be somewhat generic, reflecting the function
of the device and not its precise programming model."

Do as suggested in the binding example.
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

Documentation/devicetree/bindings/rtc/nxp,rtc-2123.txt

@@ -9,7 +9,7 @@ Optional properties:
 
 Example:
 
-rtc: nxp,rtc-pcf2123@3 {
+pcf2123: rtc@3 {
 	compatible = "nxp,rtc-pcf2123"
 	reg = <3>
 	spi-cs-high;