tests: arch: arm_no_multithreading: Run on mps3_an547 board

This commit updates the `arm_no_multithreading` test to run on the
recently added `mps3_an547` board.

The `mps3_an547` SoC includes the Cortex-M55 processor which implements
the ARMv8.1-M architecture that is not covered by any of the boards in
the current "allowed platforms" list for this test.

Signed-off-by: Stephanos Ioannidis <[email protected]>

Author: stephanosio

tests/arch/arm/arm_no_multithreading/testcase.yaml

@@ -6,4 +6,4 @@ tests:
   arch.arm.no_multithreading:
     filter: not CONFIG_TRUSTED_EXECUTION_NONSECURE
     platform_allow: qemu_cortex_m0 qemu_cortex_m3 mps2_an385 mps2_an521
-      nrf52840dk_nrf52840 nrf9160dk_nrf9160 nrf51dk_nrf51422
+      mps3_an547 nrf52840dk_nrf52840 nrf9160dk_nrf9160 nrf51dk_nrf51422