docs(maintainers): add missing ST files

The files under tools/fiptool/plat_fiptool/st/ directory were not listed
as files maintained by STMicroelectronics.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I4120368253447d4dadc4ce4b6957ffbe6310da86
diff --git a/docs/about/maintainers.rst b/docs/about/maintainers.rst
index a89eecb..3a8fd81 100644
--- a/docs/about/maintainers.rst
+++ b/docs/about/maintainers.rst
@@ -813,6 +813,7 @@
 :|F|: include/drivers/st/
 :|F|: include/dt-bindings/\*/stm32\*
 :|F|: plat/st/
+:|F|: tools/fiptool/plat_fiptool/st/
 :|F|: tools/stm32image/
 
 Synquacer platform port