doc: spm: update messaging method field

As per FF-A v1.0 spec, Table 3.1, messaging method field also contains
information about whether partition supports managed exit or not.
Since a partition can support managed exit only if it supports direct
messaging, so there are two new possible values, managed exit with only
direct messaging or with both messaging methods.

Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Change-Id: Ic77cfb37d70975c3a36c56f8b7348d385735f378
1 file changed