dt-binding: cpu-topology: Move cpu-map to a common binding.

cpu-map binding can be used to described cpu topology for both
RISC-V & ARM. It makes more sense to move the binding to document
to a common place.

The relevant discussion can be found here.
https://lkml.org/lkml/2018/11/6/19

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>