dt-bindings: arm: move cpu-capacity to a shared loation

RISC-V uses the same generic topology code as arm64 & while there
currently exists no binding for cpu-capacity on RISC-V, the code paths
can be hit if the property is present.

Move the documentation of cpu-capacity to a shared location, ahead of
defining a binding for capacity-dmips-mhz on RISC-V. Update some
references to this document in the process.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Ley Foon Tan <leyfoon.tan@starfivetech.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Yanteng Si <siyanteng@loongson.cn>
Link: https://lore.kernel.org/r/20230104180513.1379453-2-conor@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>