PSCI: Add documentation and fix plat_is_my_cpu_primary()

This patch adds the necessary documentation updates to porting_guide.md
for the changes in the platform interface mandated as a result of the new
PSCI Topology and power state management frameworks. It also adds a
new document `platform-migration-guide.md` to aid the migration of existing
platform ports to the new API.

The patch fixes the implementation and callers of
plat_is_my_cpu_primary() to use w0 as the return parameter as implied by
the function signature rather than x0 which was used previously.

Change-Id: Ic11e73019188c8ba2bd64c47e1729ff5acdcdd5b

1 files changed, 0 insertions, 0 deletions

diff --git a/docs/diagrams/psci-suspend-sequence.png b/docs/diagrams/psci-suspend-sequence.png
new file mode 100644
index 00000000..1703ea68
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+++ b/docs/diagrams/psci-suspend-sequence.png