ARM: dts: Improve CCI bindings documentation

Improve the description of the value types used in the CCI binding.

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>
Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>