tools/docker: Dockerfile: Don't specify dtc submodule

When building qemu, all required submodules (of which we need more than
just dtc) are handled automatically. Currently trying to init the
submodule the way we do results in a git failure.

Reported-by: Alexey Brodkin <Alexey.Brodkin@synopsys.com>
Signed-off-by: Tom Rini <trini@konsulko.com>