Documentation: devicetree: standardize/consolidate on "wakeup-source" property

Currently different drivers use multiple forms of annotating devices
that should be set up as wakeup sources for the system.

This patch adds a separate binding document inorder to standardize and
consolidate to use "wakeup-source" boolean property to mark the devices
as wakeup capable.

Cc: Rob Herring <robh+dt@kernel.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>