Fix DTB generation mechanism and build failure

Fix DTB generation mechanism and build failure

I ran into a build failure with --enable-targets=all due to the fact that
the moxie sim expects to be able to use the dtc tool.  If it isn't available,
the builds fails.

The following patch adds a prebuilt dtb file to the tree. That file is the one
that is used for installations.

The patch also enables (re-)generation of the dtb file through maintainer
mode, if it needs to be updated due to a change in the dts file.

Tested on aarch64-linux/x86_64-linux.

sim/moxie/ChangeLog:

2021-04-08  Luis Machado  <luis.machado@linaro.org>

	* Makefile.in (moxie-gdb.dtb): Add maintainer mode dependency.
	(install-dtb): Install prebuilt dtb file.
	* moxie-gdb.dtb: New prebuilt file.