psci: finish psci_present implementation

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. While we're here, convert it to a
boolean.

Reviewed by: andrew (slightly earlier version)
Differential Revision: https://reviews.freebsd.org/D34553