ARM: PSCI: Enable the PSCI node

When fixing up the DT to report PSCI support, explicitly enable the node.
DTs may ship with the node disabled in case a PSCI implementation is not
present, and expect any PSCI implementation to enable the node if they are
actually present.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>