[µTVM] Zephyr: Add MPS2-AN521 board as a test platform (apache#7864)

Now that MPS2-AN521 board is supported as a µTVM target, add it as test
platform so tests can run against it by using:

$ pytest test_zephyr.py --microtvm-platforms=mps2_an521

Signed-off-by: Gustavo Romero <gustavo.romero@linaro.org>

tests/micro/zephyr/conftest.py

@@ -27,6 +27,7 @@
     "stm32f746xx_nucleo": ("stm32f746xx", "nucleo_f746zg"),
     "stm32f746xx_disco": ("stm32f746xx", "stm32f746g_disco"),
     "nrf5340dk": ("nrf5340dk", "nrf5340dk_nrf5340_cpuapp"),
+    "mps2_an521": ("mps2_an521", "mps2_an521-qemu"),
 }