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>