Platform: Add support for MPS3 AN524 FPGA board

Details:
1. Adds AN524 platform support files
2. Adds a new target platform to the build system: "AN524"
3. Updates the docs with details for AN524

Change-Id: I7c192e951253dfc72512d73ee5da3f94023a248f
Signed-off-by: Kevin Peng <kevin.peng@arm.com>
diff --git a/docs/user_guides/tfm_build_instruction.rst b/docs/user_guides/tfm_build_instruction.rst
index fd21f7b..6c89699 100644
--- a/docs/user_guides/tfm_build_instruction.rst
+++ b/docs/user_guides/tfm_build_instruction.rst
@@ -33,6 +33,8 @@
             ``-DTARGET_PLATFORM=MUSCA_A``
           - Musca-B1 test chip board (Cortex-M33 SSE-200 subsystem)
             ``-DTARGET_PLATFORM=MUSCA_B1``
+          - Cortex-M33 SSE-200 subsystem for MPS3 (AN524)
+            ``-DTARGET_PLATFORM=AN524``
 
    * - -DCOMPILER=<compiler name>
      - Specifies the compiler toolchain