doc: Document Branch Protection

Add Branch Procection related information to the documentation.

Signed-off-by: Gabor Toth <gabor.toth2@arm.com>
Signed-off-by: Gyorgy Szing <gyorgy.szing@arm.com>
Change-Id: Ic67876afc577a28ec6b63ebe85204bb8e3239d30
2 files changed