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_build_instruction.rst b/docs/user_guides/tfm_build_instruction.rst
index 70c5358..db6e0fe 100644
--- a/docs/user_guides/tfm_build_instruction.rst
+++ b/docs/user_guides/tfm_build_instruction.rst
@@ -202,10 +202,14 @@
             ``-DTARGET_PLATFORM=AN521``
           - Cortex-M23 IoT Kit subsystem for MPS2+ (AN519)
             ``-DTARGET_PLATFORM=AN519``
+          - Arm SSE-123 Example Subsystem for MPS2+ (AN539)
+            ``-DTARGET_PLATFORM=AN539``
           - Musca-A test chip board (Cortex-M33 SSE-200 subsystem)
             ``-DTARGET_PLATFORM=MUSCA_A``
           - Musca-B1 test chip board (Cortex-M33 SSE-200 subsystem)
             ``-DTARGET_PLATFORM=MUSCA_B1``
+          - Musca-S1 test chip board (Cortex-M33 SSE-200 subsystem)
+            ``-DTARGET_PLATFORM=MUSCA_S1``
           - Cortex-M33 SSE-200 subsystem for MPS3 (AN524)
             ``-DTARGET_PLATFORM=AN524``