Add UEFI SMM service documentation

Adds developer documentation for the smm variable service
provider and the smm-gateway deployment.

Signed-off-by: Julian Hall <julian.hall@arm.com>
Change-Id: Iba3216986831afe22ce62a51a92ba53c4920f8db
diff --git a/docs/developer/service-descriptions/index.rst b/docs/developer/service-descriptions/index.rst
index 5962e37..6574a3e 100644
--- a/docs/developer/service-descriptions/index.rst
+++ b/docs/developer/service-descriptions/index.rst
@@ -8,9 +8,10 @@
     attest-service-description
     crypto-service-description
     secure-storage-service-description
+    uefi-smm-services
 
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-*Copyright (c) 2020-2021, Arm Limited and Contributors. All rights reserved.*
+*Copyright (c) 2020-2022, Arm Limited and Contributors. All rights reserved.*
 
 SPDX-License-Identifier: BSD-3-Clause