ARM: dts: Document the CCI PMU DT bindings

The CCI PMU can profile bus transactions at the master and slave
interfaces of the CCI. The PMU can be used to observe an aggregated
view of the bus traffic between the various components connected to the
CCI.

Introduce a binding for the CCI PMU. The PMU node will be a sub-node of
the CCI node.

Cc: devicetree@vger.kernel.org
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Nicolas Pitre <nicolas.pitre@linaro.org>
Cc: Rob Herring <rob.herring@calxeda.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Kumar Gala <galak@codeaurora.org>
Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>