mips: lantiq: danube: rename stp node on EASY50712 reference board

This fixes the following warning:
arch/mips/boot/dts/lantiq/danube_easy50712.dtb: stp@e100bb0 (lantiq,gpio-stp-xway): $nodename:0: 'stp@e100bb0' does not match '^gpio@[0-9a-f]+$'
from schema $id: http://devicetree.org/schemas/gpio/gpio-stp-xway.yaml#

Signed-off-by: Aleksander Jan Bajkowski <[e-mail]>
Signed-off-by: Thomas Bogendoerfer <[e-mail]>

arch/mips/boot/dts/lantiq/danube_easy50712.dts

···
 			lantiq,tx-burst-length = <4>;
 		};
 
-		stp0: stp@e100bb0 {
+		stp0: gpio@e100bb0 {
 			#gpio-cells = <2>;
 			compatible = "lantiq,gpio-stp-xway";
 			gpio-controller;