docs(msm8916): document new platforms

Document the new platform build options for the MSM8916 port which now
supports multiple similar Qualcomm SoCs:

  - Snapdragon 410 (PLAT=msm8916) as before
  - Snapdragon 615 (PLAT=msm8939)
  - Snapdragon 210 (PLAT=msm8909)
  - Snapdragon X5 Modem (PLAT=mdm9607)

The latter two use AArch32-only ARM Cortex-A7 cores that only support
using BL32/SP_MIN and not BL31 on AArch64.

Change-Id: I9fffe60dd0ad2acc18f006f11e91854b9e8dcb8f
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
diff --git a/docs/about/maintainers.rst b/docs/about/maintainers.rst
index 663859f..6733d66 100644
--- a/docs/about/maintainers.rst
+++ b/docs/about/maintainers.rst
@@ -698,7 +698,10 @@
 :|M|: Nikita Travkin <nikita@trvn.ru>
 :|G|: `TravMurav`_
 :|F|: docs/plat/qti-msm8916.rst
+:|F|: plat/qti/mdm9607/
+:|F|: plat/qti/msm8909/
 :|F|: plat/qti/msm8916/
+:|F|: plat/qti/msm8939/
 
 Raspberry Pi 3 platform port
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^