ARM: dts: imx27-phytec-phycore-som: Use 'rtc' as node name

Node names should be generic.

Use 'rtc' for the rtc node to fix the following dt-schema warning:

imx27-phytec-phycore-rdk.dtb: pcf8563@51: $nodename:0: 'pcf8563@51' does not match '^rtc(@.*|-([0-9]|[1-9][0-9]+))?$'
	from schema $id: http://devicetree.org/schemas/rtc/nxp,pcf8563.yaml#Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/nxp/imx/imx27-phytec-phycore-som.dtsi

@@ -180,7 +180,7 @@ eeprom@52 {
 		reg = <0x52>;
 	};
 
-	pcf8563@51 {
+	rtc@51 {
 		compatible = "nxp,pcf8563";
 		reg = <0x51>;
 	};