devicetree: bindings: Document qcom,saw2 node

The saw2 binding describes the SPM/AVS wrapper hardware used to
control the regulator supplying voltage to the Krait CPUs.

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