dt-bindings: arm: Add some constraints for PSCI nodes

There's no point allowing anything else but "psci" for the corresponding
nodename of the PSCI node, so let's make this an explicit requirement.

Moreover, let's also prevent additional properties, which also allows a
better verification of the "patternProperties" bindings for subnodes.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>