PM / doc: Update device documentation for devices in IRQ-safe PM domains

into a low-power state together at the same time by turning off the shared

power resource.  Of course, they also need to be put into the full-power state

together, by turning the shared power resource on.  A set of devices with this

property is often referred to as a power domain.

property is often referred to as a power domain. A power domain may also be

nested inside another power domain. The nested domain is referred to as the

sub-domain of the parent domain.

Support for power domains is provided through the pm_domain field of struct

device.  This field is a pointer to an object of type struct dev_pm_domain,

modifying the platform bus type.  Other platforms need not implement it or take

it into account in any way.

Devices may be defined as IRQ-safe which indicates to the PM core that their

runtime PM callbacks may be invoked with disabled interrupts (see

Documentation/power/runtime_pm.txt for more information).  If an IRQ-safe

device belongs to a PM domain, the runtime PM of the domain will be

disallowed, unless the domain itself is defined as IRQ-safe. However, it

makes sense to define a PM domain as IRQ-safe only if all the devices in it

are IRQ-safe. Moreover, if an IRQ-safe domain has a parent domain, the runtime

PM of the parent is only allowed if the parent itself is IRQ-safe too with the

additional restriction that all child domains of an IRQ-safe parent must also

be IRQ-safe.

Device Low Power (suspend) States

---------------------------------