kbuild: turn of dtc unit address warnings by default

DTC 1.4.2 or later checks DT unit-address without reg property and
vice-versa, and generates lots of warnings.  Fixing DT files will
take for a while.  Until then, let's turn off the check unless
building with W=*.

Introduce a new helper dtc-option to check if the option is supported
in order to suppress warnings on older versions.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
index 6547e57..7b2cffc 100644
--- a/scripts/Makefile.extrawarn
+++ b/scripts/Makefile.extrawarn
@@ -57,4 +57,10 @@
 endif
 
 KBUILD_CFLAGS += $(warning)
+
+else
+
+# Disable noisy checks by default
+DTC_FLAGS += $(call dtc-option,-Wno-unit_address_vs_reg)
+
 endif