Documentation: arm: Link idle-states binding to "enable-method" property

The "enable-method" property for cpu nodes needs to be "psci" for CPU
idle management to be setup correctly.

Add a note to the binding documentation to this effect to make it
obvious.

Signed-off-by: Amit Kucheria <amit.kucheria@linaro.org>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>