devicetree: bindings: Document qcom,kpss-acc

The kpss acc binding describes the clock, reset, and power domain
controller for a Krait CPU.

Cc: <devicetree@vger.kernel.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Kumar Gala <galak@codeaurora.org>