docs(spm): update ff-a boot protocol documentation

Updated following sections to document implementation of the FF-A boot
information protocol:
- Describing secure partitions.
- Secure Partition Packages.
- Passing boot data to the SP.
Also updated description of the manifest field 'gp-register-num'.

Signed-off-by: J-Alves <joao.alves@arm.com>
Change-Id: I5c856437b60cdf05566dd636a01207c9b9f42e61