docs(sdei): provide security guidelines when using SDEI

Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Signed-off-by: Jayanth Dodderi Chidanand <jayanthdodderi.chidanand@arm.com>
Change-Id: Ic27bdc88186f6805adee2f452503856e213a4710
1 file changed