docs(spm-mm): remove reference to SEL2 SPMC

As the SEL2 SPMC design doc is migrated to Hafnium tree, remove the
reference to this implementation from TF-A's SPM-MM doc.

Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
Change-Id: I1609c7d1d098420412dffc7b1309cc9c11502f8a
1 file changed