docs(spm): add memory sharing documentation

Add documentation that explains implementation specific
relevant information from the update done to FF-A v1.1
memory sharing in Hafnium.

Change-Id: Ifc3c6b86c0545d53331207b017b990427ee84f2d
Signed-off-by: J-Alves <joao.alves@arm.com>
1 file changed