Add device bindings for CPUs to suspend using PSCI as the enable-method.

Cc: <devicetree@vger.kernel.org>
Signed-off-by: NLina Iyer <lina.iyer@linaro.org>
Tested-by: NAndy Gross <andy.gross@linaro.org>
Signed-off-by: NAndy Gross <andy.gross@linaro.org>