The `psci_plat_pm_ops` global pointer is initialized during cold boot by the
primary CPU and will be accessed by the secondary CPUs before enabling data
cache during warm boot. This patch adds a missing data cache flush of
`psci_plat_psci_ops` after initialization during psci_setup() so that
secondaries can see the updated `psci_plat_psci_ops` pointer.

Fixes ARM-software/tf-issues#424

Change-Id: Id4554800b5646302b944115a33be69507d53cedb