KVM: selftests: Update top-of-file comment in psci_test

Fix the comment to accurately describe the test and recently added
SYSTEM_SUSPEND test case.

What was once psci_cpu_on_test was renamed and extended to squeeze in a
test case for PSCI SYSTEM_SUSPEND. Nonetheless, the author of those
changes (whoever they may be...) failed to update the file comment to
reflect what had changed.

Reported-by: Reiji Watanabe <reijiw@google.com>
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220819162100.213854-1-oliver.upton@linux.dev