buildman: Detect dtc warnings

At present messages from the device-tree compiler like this:

  arch/arm/dts/socfpga_arria10_socdk_sdmmc.dtb: Warning
     (avoid_unnecessary_addr_size): /clocks: unnecessary
     #address-cells/#size-cells without "ranges" or child "reg" property

are detected as errors since they don't match the gcc warning regex. Add a
new one for dtc to fix this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/test.py b/tools/buildman/test.py
index 61a4626..5c82c50 100644
--- a/tools/buildman/test.py
+++ b/tools/buildman/test.py
@@ -46,8 +46,9 @@
 make: *** [common/libcommon.o] Error 2
 Make failed
 ''',
-    '''main.c: In function 'main_loop3':
-main.c:280:6: warning: unused variable 'mary' [-Wunused-variable]
+    '''arch/arm/dts/socfpga_arria10_socdk_sdmmc.dtb: Warning \
+(avoid_unnecessary_addr_size): /clocks: unnecessary #address-cells/#size-cells \
+without "ranges" or child "reg" property
 ''',
     '''powerpc-linux-ld: warning: dot moved backwards before `.bss'
 powerpc-linux-ld: warning: dot moved backwards before `.bss'