docs(maintainers): add code owner to sptool

Add Joao Alves as code owner to the sptool.

Signed-off-by: J-Alves <joao.alves@arm.com>
Change-Id: I9e44e322ba1cce62308bf16c4a6253f7b0117fe0
diff --git a/docs/about/maintainers.rst b/docs/about/maintainers.rst
index 159a3db..871afe3 100644
--- a/docs/about/maintainers.rst
+++ b/docs/about/maintainers.rst
@@ -787,6 +787,8 @@
 ^^^^^^
 :|M|: Manish Pandey <manish.pandey2@arm.com>
 :|G|: `manish-pandey-arm`_
+:|M|: Joao Alves <Joao.Alves@arm.com>
+:|G|: `J-Alves`_
 :|F|: tools/sptool/
 
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