Header file for MHL HAL interface definition

Added MHL(Mobile High-definition Link) HAL definition to provide
interface between the hardware driver and the HdmiControlService
which also talks to HDMI-CEC HAL. They are both based on HDMI signal
specification.

MHL version 2.0 and above is supported.

Change-Id: Idcdfa1a61abfbf63bf838f6efe8b037fb32764ee