Documentation: DT: arm: define CPU topology bindings

The advent of multi-cluster ARM systems requires a mechanism to describe
how in hierarchical terms CPUs are connected in ARM SoCs so that the kernel
can initialize and map resources like IRQs and memory space to specific
group(s) of CPUs.

The CPU topology is made up of multiple hierarchy levels whose bottom
layers (aka leaf nodes in device tree syntax) contain links to the HW
CPUs in the system.

The topology bindings are generic for both 32-bit and 64-bit systems and
lay the groundwork on top of which affinity schemes can be built.

This patch provides the documentation in the kernel required to define the
device tree bindings describing the CPU topology for ARM 32-bit and 64-bit
systems.

Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>