ci: Add notes about untested toolchains

This commit adds the notes about the untested Xtensa toolchain variants.

These Xtensa toolchain variants currently do not have an upstream user
platform and therefore cannot be tested using twister.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

.github/workflows/ci.yml

@@ -1559,6 +1559,10 @@ jobs:
             xtensa-intel_tgl_adsp_zephyr-elf)
               PLATFORM_ARGS+="-p intel_adsp/cavs25 "
               ;;
+            xtensa-mtk_mt8195_adsp_zephyr-elf)
+              # xtensa-mtk_mt8195_adsp_zephyr-elf is untested because no
+              # upstream user platform is currently available.
+              ;;
             xtensa-nxp_imx_adsp_zephyr-elf)
               PLATFORM_ARGS+="-p imx8qm_mek/mimx8qm6/adsp "
               ;;
@@ -1571,6 +1575,10 @@ jobs:
             xtensa-nxp_rt500_adsp_zephyr-elf)
               PLATFORM_ARGS+="-p mimxrt595_evk/mimxrt595s/f1 "
               ;;
+            xtensa-nxp_rt600_adsp_zephyr-elf)
+              # xtensa-nxp_rt600_adsp_zephyr-elf is untested because no
+              # upstream user platform is currently available.
+              ;;
             xtensa-sample_controller_zephyr-elf)
               PLATFORM_ARGS+="-p qemu_xtensa "
               ;;