Document the compatible string for the Kryo family of qcom cpus.

Cc: <devicetree@vger.kernel.org>
Signed-off-by: NStephen Boyd <sboyd@codeaurora.org>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NAndy Gross <andy.gross@linaro.org>