devicetree: bindings: Document qcom,idle-states

Document cpuidle states of QCOM cpus. In addition to arm-idle-state
compatible string, the ARM idle state definition must define one of the
following compatible strings -
	"qcom,idle-state-ret",
	"qcom,idle-state-spc",
	"qcom,idle-state-pc",

The compatibles helps the SPM platform driver to use the correct idle
function when the index to the idle state is passed to the platform
driver.

Signed-off-by: Lina Iyer <lina.iyer@linaro.org>