scripts/Makefile.lib: Re-add -Wno-simple_bus_reg to DTC_FLAGS

This exists in Linux Kernel with commit 70523a3ce5ff so put it in the
list of DTC_FLAGS that mirror Linux as we will catch up there.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Masahiro Yamada <masahiroy@kernel.org>