ARM: 8570/2: Documentation: devicetree: Add PL310 PM bindings

Document the DT bindings for controlling ARM PL310 Power Control
settings.

Discussion on the binding wording:
http://archive.arm.linux.org.uk/lurker/message/20160427.143444.5141d302.en.html

Signed-off-by: Brad Mouring <brad.mouring@ni.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

Documentation/devicetree/bindings/arm/l2c2x0.txt

@@ -84,6 +84,12 @@ Optional properties:
 - prefetch-instr : Instruction prefetch. Value: <0> (forcibly disable),
   <1> (forcibly enable), property absent (retain settings set by
   firmware)
+- arm,dynamic-clock-gating : L2 dynamic clock gating. Value: <0> (forcibly
+  disable), <1> (forcibly enable), property absent (OS specific behavior,
+  preferrably retain firmware settings)
+- arm,standby-mode: L2 standby mode enable. Value <0> (forcibly disable),
+  <1> (forcibly enable), property absent (OS specific behavior,
+  preferrably retain firmware settings)
 
 Example: