docs: update PSCI reference

PSCI specification reference in the documentation is updated
to point to latest specification and duplicate PSCI references are
removed.

Change-Id: I35ee365f08c557f3017af4d51f6d063a7501b27e
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
6 files changed