Platform: Add support for MPS2 AN539 FPGA board

Adds AN539 platform support files. Adds a new target platform to
the build system: "AN539".

Change-Id: I27336502f9526b0e5476d0d594c2d24fb73e72dc
Signed-off-by: Marton Berke <marton.berke@arm.com>
diff --git a/docs/user_guides/tfm_secure_boot.rst b/docs/user_guides/tfm_secure_boot.rst
index a10d7d3..9aed8dd 100644
--- a/docs/user_guides/tfm_secure_boot.rst
+++ b/docs/user_guides/tfm_secure_boot.rst
@@ -213,6 +213,8 @@
 +----------+-----------------+---------------+----------+-------------+-----------------+
 | AN519    | Yes             | Yes           | Yes      | Yes         | No              |
 +----------+-----------------+---------------+----------+-------------+-----------------+
+| AN539    | Yes             | Yes           | Yes      | Yes         | No              |
++----------+-----------------+---------------+----------+-------------+-----------------+
 | Musca-A  | No              | No            | No       | No          | Yes             |
 +----------+-----------------+---------------+----------+-------------+-----------------+
 | Musca-B1 | Yes             | No            | No       | Yes         | No              |