dt-bindings: powerpc: Add a schema for the 'sleep' property

Document the PowerPC specific 'sleep' property as a schema. It is
currently only documented in booting-without-of.rst which is getting
removed.

Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201008142420.2083861-1-robh@kernel.orgSigned-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/powerpc/sleep.yaml

+# SPDX-License-Identifier: GPL-2.0-only
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/powerpc/sleep.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: PowerPC sleep property
+
+maintainers:
+  - Rob Herring <robh@kernel.org>
+
+description: |
+  Devices on SOCs often have mechanisms for placing devices into low-power
+  states that are decoupled from the devices' own register blocks.  Sometimes,
+  this information is more complicated than a cell-index property can
+  reasonably describe.  Thus, each device controlled in such a manner
+  may contain a "sleep" property which describes these connections.
+
+  The sleep property consists of one or more sleep resources, each of
+  which consists of a phandle to a sleep controller, followed by a
+  controller-specific sleep specifier of zero or more cells.
+
+  The semantics of what type of low power modes are possible are defined
+  by the sleep controller.  Some examples of the types of low power modes
+  that may be supported are:
+
+   - Dynamic: The device may be disabled or enabled at any time.
+   - System Suspend: The device may request to be disabled or remain
+     awake during system suspend, but will not be disabled until then.
+   - Permanent: The device is disabled permanently (until the next hard
+     reset).
+
+  Some devices may share a clock domain with each other, such that they should
+  only be suspended when none of the devices are in use.  Where reasonable,
+  such nodes should be placed on a virtual bus, where the bus has the sleep
+  property.  If the clock domain is shared among devices that cannot be
+  reasonably grouped in this manner, then create a virtual sleep controller
+  (similar to an interrupt nexus, except that defining a standardized
+  sleep-map should wait until its necessity is demonstrated).
+
+select: true
+
+properties:
+  sleep:
+    $ref: /schemas/types.yaml#definitions/phandle-array
+
+additionalProperties: true