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This patch corrects some typos in the platform migration guide. More                                                                                                                                                                                                              
importantly, the commit ID of the patch that implements migration of ARM
Reference platforms to the new platform API has been corrected.

Change-Id: Ib0109ea42c3d2bad2c6856ab725862652da7f3c8

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