ARM: dts: imx25: Fix the iim compatible string

Per imx-iim.yaml, the compatible string should only contain a single
entry.

Use it as "fsl,imx25-iim" to fix the following dt-schema warning:

imx25-karo-tx25.dtb: efuse@53ff0000: compatible: ['fsl,imx25-iim', 'fsl,imx27-iim'] is too long
	from schema $id: http://devicetree.org/schemas/nvmem/imx-iim.yaml#Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/nxp/imx/imx25.dtsi

@@ -552,7 +552,7 @@ pwm1: pwm@53fe0000 {
 			};
 
 			iim: efuse@53ff0000 {
-				compatible = "fsl,imx25-iim", "fsl,imx27-iim";
+				compatible = "fsl,imx25-iim";
 				reg = <0x53ff0000 0x4000>;
 				interrupts = <19>;
 				clocks = <&clks 99>;