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>