intel_adsp: dts: ace: lower case 71C00 to fix DTC warning

Fixes the warning below. This commit does not change the firmware
binary. Thanks Kumar Gala for the suggestion.

  build-mtl/zephyr/zephyr.dts:279.42-285.5: Warning (simple_bus_reg):
  /soc/ace_comm_widget@71C00: simple-bus unit address format error,
  expected "71c00"

Signed-off-by: Marc Herbert <marc.herbert@intel.com>