usb: add bus type for USB ULPI

UTMI+ Low Pin Interface (ULPI) is a commonly used PHY
interface for USB 2.0. The ULPI specification describes a
standard set of registers which the vendors can extend for
their specific needs. ULPI PHYs provide often functions
such as charger detection and ADP sensing and probing.

There are two major issues that the bus type is meant to
tackle:

Firstly, ULPI registers are accessed from the controller.
The bus provides convenient method for the controller
drivers to share that access with the actual PHY drivers.

Secondly, there are already platforms that assume ULPI PHYs
are runtime detected, such as many Intel Baytrail based
platforms. They do not provide any kind of hardware
description for the ULPI PHYs like separate ACPI device
object that could be used to enumerate a device from.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Acked-by: David Cohen <david.a.cohen@linux.intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>