Documentation: arm: add cache DT bindings

On ARM systems the cache topology cannot be probed at runtime, in
particular, it is impossible to probe which CPUs share a given cache
level. Power management software requires this knowledge to implement
optimized power down sequences, hence this patch adds a document that
defines the DT cache bindings for ARM systems. The bindings are compliant
with ePAPR (PowerPC bindings), even though most of the cache nodes
properties requirements are overriden, because caches geometry for
architected caches is probeable on ARM systems. This patch also adds
properties that are specific to ARM architected caches to the existing ones
defined in the ePAPR v1.1, as bindings extensions.

Change-Id: I37ca3aae0471fcd60499615df77093d5b5451bf8
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Abhimanyu Kapur <abhimany@codeaurora.org>