dt-bindings: clock: Document qcom,krait-cc

The Krait clock controller controls the krait CPU and the L2 clocks
consisting a primary mux and secondary mux. Add document for that.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Sricharan R <sricharan@codeaurora.org>
Tested-by: Craig Tatlor <ctatlor97@gmail.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>