dts: calxeda: add ARM PSCI binding

Add the PSCI binding node for Calxeda SOCs. Only claiming "arm,psci"
support since there is no agreement on 0.2 binding definition.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Stephen Warren <swarren@wwwdotorg.org>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: devicetree@vger.kernel.org