ARM: dts: imx27: Use 'bus' for AIPI bus

Per simple-bus.yaml, 'aipi' is not a valid node name.

Change it to 'bus' to fix the following dt-schema warning:

imx27-pdk.dtb: aipi@10020000: $nodename:0: 'aipi@10020000' does not match '^([a-z][a-z0-9\\-]+-bus|bus|localbus|soc|axi|ahb|apb)(@.+)?$'
	from schema $id: http://devicetree.org/schemas/simple-bus.yaml#Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/nxp/imx/imx27.dtsi

@@ -81,7 +81,7 @@ soc: soc {
 		interrupt-parent = <&aitc>;
 		ranges;
 
-		aipi1: aipi@10000000 { /* AIPI1 */
+		aipi1: bus@10000000 { /* AIPI1 */
 			compatible = "fsl,aipi-bus", "simple-bus";
 			#address-cells = <1>;
 			#size-cells = <1>;
@@ -453,7 +453,7 @@ gpt6: timer@1001f000 {
 			};
 		};
 
-		aipi2: aipi@10020000 { /* AIPI2 */
+		aipi2: bus@10020000 { /* AIPI2 */
 			compatible = "fsl,aipi-bus", "simple-bus";
 			#address-cells = <1>;
 			#size-cells = <1>;