thunderbolt: Add functions to support DisplayPort bandwidth allocation mode

USB4 spec defines an additional feature that DP IN adapters can
implement (alongside with the graphics DPCD register set) to support
more dynamic bandwidth management for DisplayPort tunnels. For the
connection manager the communication happens through the DP IN adapter
using a set of registers in the adapter config space allocated for this.
Add functions that export this functionality for the rest of the driver.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>