power: supply: Add 'usb_type' property and supporting code

This commit adds the 'usb_type' property to represent USB supplies
which can report a number of different types based on a connection
event.

Examples of this already exist in drivers whereby the existing 'type'
property is updated, based on an event, to represent what was
connected (e.g. USB, USB_DCP, USB_ACA, ...). Current implementations
however don't show all supported connectable types, so this knowledge
has to be exlicitly known for each driver that supports this.

The 'usb_type' property is intended to fill this void and show users
all possible USB types supported by a driver. The property, when read,
shows all available types for the driver, and the one currently chosen
is highlighted/bracketed. It is expected that the 'type' property
would then just show the top-level type 'USB', and this would be
static.

Currently the 'usb_type' enum contains all of the USB variant types
that exist for the 'type' enum at this time, and in addition has
SDP and PPS types. The mirroring is intentional so as to not impact
existing usage of the 'type' property.

Signed-off-by: Adam Thomson <Adam.Thomson.Opensource@diasemi.com>
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>