fix(docs): add missing line in the fiptool command for stm32mp1

Add the missing trusted key certificate in the fiptool
command line.

Signed-off-by: Lionel Debieve <lionel.debieve@foss.st.com>
Change-Id: Ife95b0261f04b7fd07a9b01488f9e5be9b87e841
1 file changed