ARM: mxs: add dtb-y target into Makefile.boot

Add dtb-y target into Makefile.boot, so that "make ARCH=arm dtbs" can
build out all the dtbs listed there.  It's very useful for build-testing
all the <board>.dts when imx28.dtsi changes.
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>

arch/arm/mach-mxs/Makefile.boot

 zreladdr-y += 0x40008000
+
+dtb-y += apx4devkit.dtb \
+	 cfa10036.dtb \
+	 imx23-evk.dtb \
+	 imx23-olinuxino.dtb \
+	 imx28-evk.dtb \
+	 m28evk.dtb \
+	 stmp378x_devb.dtb \
+	 tx28.dtb \