Makefile: Update minimum dtc version to 1.4.3

With support for overlays and calling the -@ flag to dtc we need to have
at least 1.4.3 available now.

Cc: Simon Glass <sjg@chromium.org>
Reported-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Tom Rini <trini@konsulko.com>