Docs: Add missing platforms to documentation

Add missing documentation for Musca-S1 and AN539

Change-Id: Ic98d3a60c3ea89513a0b5b8cbf6ad6e117506162
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 4adc1e6..3c63258 100644
--- a/docs/user_guides/tfm_secure_boot.rst
+++ b/docs/user_guides/tfm_secure_boot.rst
@@ -235,6 +235,8 @@
 +----------+-----------------+---------------+----------+-------------+-----------------+
 | Musca-B1 | Yes             | No            | No       | Yes         | No              |
 +----------+-----------------+---------------+----------+-------------+-----------------+
+| Musca-S1 | Yes             | No            | No       | Yes         | No              |
++----------+-----------------+---------------+----------+-------------+-----------------+
 | AN524    | Yes             | No            | No       | Yes         | No              |
 +----------+-----------------+---------------+----------+-------------+-----------------+