arm: mps_an521: Add support for the AN521 on MPS2+

The AN521 is a Soft Macro Model implementation of the SSE-200 subsystem
with SIE-200 and CMSDK components targeting the MPS2+ board. The
SSE-200 subsystem implements two Cortex-M33 cores.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>
Signed-off-by: Karl Zhang <karl.zhang@linaro.org>