Add dtb/zynq to generate dtb files for Zynq-based boards

Add dtb/zynq to generate dtb files for Zynq-based boards

It seems that the only way to supply dtb to loader on Zynq-based
SoCs is to manually generate dtb and place it to pre-defined location
on SD card or TFTP server where loader can pick it up.  More modern
approach is to add modules/dtb/%soc% module and let installworld
target generate dtb and copy them to /boot/dtb/ where they can be
loaded by ubldr