greybus: Documentation/sysfs: add example control devices

Add control devices to the example sysfs tree.

Control devices are named

	<bus_id>-<module_id>.<intf_id>.ctrl

and expose attributes that are specific to the greybus interface type.
Specifically, dummy interfaces do not have a control device.

Currently, only the vendor and product strings extracted from the
manifest are exported.

A subtree of the example tree now looks as follows:

greybus1/
├── 1-5
│   ├── 1-5.5
│   │   ├── 1-5.5.2
│   │   │   ├── bundle_class
│   │   │   ├── bundle_id
│   │   │   └── state
│   │   ├── 1-5.5.ctrl
│   │   │   ├── product_string
│   │   │   └── vendor_string
│   │   ├── ddbl1_manufacturer_id
│   │   ├── ddbl1_product_id
│   │   ├── interface_id
│   │   ├── product_id
│   │   ├── serial_number
│   │   └── vendor_id
│   ├── 1-5.6
│   │   └── interface_id
│   ├── eject
│   ├── module_id
│   └── num_interfaces
└── 1-svc

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>