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 <root@stephanos.io>