psci: change bootverbose string to 'PSCI 0.2 compatible'

Prior to r324754 we treated PSCI 0.2 and 1.0 as identical, and r324754
extended that to include all PSCI 1.x revisions. Change the string
emitted under bootverbose to reference '0.2 compatible' to avoid
confusion when the system includes a later PSCI rev.

Discussed with: andrew
Sponsored by: The FreeBSD Foundation