scripts/Makefile.extrawarn: Silence more DTC warnings

While our "extrawarns" logic has gotten out of sync with upstream
Kbuild, for now lets start by bringing in the latest set of DTC_FLAGS
from the Linux Kernel 5.0 to match their behavior in silencing warnings
from dtc.

Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Tom Rini <trini@konsulko.com>