boards: intel_adsp_ace15_mtpm uses xcc-clang toolchain for twister

Upgrading Xtensa toolchain for intel ACE1X boards.
Newer Xtensa toolchains support only xt-clang driver so we are
forced to switch.

Signed-off-by: Andrey Borisovich <andrey.borisovich@intel.com>

boards/xtensa/intel_adsp_ace15_mtpm/intel_adsp_ace15_mtpm.yaml

@@ -5,6 +5,7 @@ arch: xtensa
 toolchain:
   - zephyr
   - xcc
+  - xt-clang
 supported:
   - dma
 testing: