msm: mhi_dev: Add support to identify mhi control state

Support clients to query the MHI (modem host interface)
state to identify the PCIe end point configuration mode
so that clients can request data over MHI. After
the host indicates all the channels are available the
device updates the file control node to "connected" and
allow clients to start using the channels.

Change-Id: I21374cd2033903dd8cc7e3b66043cf8b7c584367
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>