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
1 file changed