features/systemready: update for SRS v3.0

Following the publication of SystemReady Requirements Specification v3.0
available at https://developer.arm.com/documentation/den0109/latest

 this
change updates terminology and steps to match the new specification.

Signed-off-by: Nuno Lopes <nuno.lopes@arm.com>
Change-Id: I5b6ce653a04626aa35d940d7d4a644006c98f36d