psci: replace secure context with suspend context

psci: replace secure context with suspend context

The secure context saved and restored across a cpu_suspend operation
can be more than just the state of the secure system registers e.g. we
also need to save the affinity level till which the cpu is being
powered down. This patch creates a suspend_context data structure
which includes the system register context. This will allow other bits
to be saved and restored as well in subsequent patches.

Change-Id: I1c1f7d25497388b54b7d6ee4fab77e8c6a9992c4