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>