msm_fb: MHL: Adding MHL api's for USB interaction.

Adding MHL api's for  device discovery and registering
callback mechanism.USB driver use these API's for
synchronization with MHL driver.

Change-Id: I0846951302e3b5f5d20d566c92c2f224528ca455
Signed-off-by: Manoj Rao <manojraj@codeaurora.org>
Signed-off-by: Abhishek Kharbanda <akharban@codeaurora.org>