bus: mhi: ep: Add support for managing MHI state machine

Add support for managing the MHI state machine by controlling the state
transitions. Only the following MHI state transitions are supported:

1. Ready state
2. M0 state
3. M3 state
4. SYS_ERR state

Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>