boards: arm: add support for MPS4 Corstone-320

Add initial support for the MPS4 Corstone-320 platform, including board
and SoC definitions. This platform features a Cortex-M85 CPU with an
Ethos-U85 NPU and runs in simulation using the FVP_Corstone_SSE-320
Fixed Virtual Platform.

Signed-off-by: Sudan Landge <sudan.landge@arm.com>