docs(ff-a): boot order field of SPs manifest

Document `boot-order` field from FF-A partitions manifest, in accordance
to Hafnium's (SPM) implementation.

Signed-off-by: J-Alves <joao.alves@arm.com>
Change-Id: I9fd070100ee52e0d465d2cce830cc91d78bddfc0
1 file changed