package/dtc: Update to v1.4.0

Currently, we're using an untagged commit of dtc. Since inclusion in
buildroot, version 1.4.0 has been released, which includes new
endian-annotated type defintions.

We want to use these type definitions in kexec-lite, so update to the
new release.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>