usb: phy: Dual role sysfs class definition

This CL adds a new class to monitor and change
dual role usb ports from userspace. The usb
phy drivers can register to the dual_role_usb
class and expose the capabilities of the ports.
The phy drivers can decide on whether a specific
attribute can be changed from userspace by
choosing to implement the appropriate callback.

Signed-off-by: Badhri Jagan Sridharan <badhri@google.com>
Bug: 21615151
Change-Id: I121b4a94437972b5abe8f781e3007efa8531978