psci: finish psci_present implementation

[kevans]

Summary

This was already declared in psci.h, but it was never defined/set. Do
this now, so we can use it to decide if enable-method in /cpus FDT nodes
should be inspected later on. Currently, we'll arbitrarily set it to
the detected or derived PSCI version -- there aren't any plans for this
particular value to be of any significance.

[andrew]

Why not make it a bool while here?

[kevans]

In D34553#783949, @andrew wrote:

Why not make it a bool while here?

Sure, I'll convert it pre-commit if that's alright.

[andrew]

Yes