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>