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: cb612390 ("ARM: dts: Only build dtb if associated Arch and/or SoC is enabled")
Cc: <stable@vger.kernel.org>