ARM: dts: fix imx27 dtb build rule

The i.MX27 dtb build should be controlled by CONFIG_SOC_IMX27 rather
than CONFIG_SOC_IMX31.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Fixes: cb612390e5 ("ARM: dts: Only build dtb if associated Arch and/or SoC is enabled")
Cc: <stable@vger.kernel.org>

arch/arm/boot/dts/Makefile

@@ -223,7 +223,7 @@ dtb-$(CONFIG_SOC_IMX25) += \
 	imx25-eukrea-mbimxsd25-baseboard-dvi-vga.dtb \
 	imx25-karo-tx25.dtb \
 	imx25-pdk.dtb
-dtb-$(CONFIG_SOC_IMX31) += \
+dtb-$(CONFIG_SOC_IMX27) += \
 	imx27-apf27.dtb \
 	imx27-apf27dev.dtb \
 	imx27-eukrea-mbimxsd27-baseboard.dtb \