bus: mhi: core: Expose mhi_get_exec_env() API for controllers

The mhi_get_exec_env() APIs can be used by the controller drivers
to query the execution environment of the MHI device. Expose it
so it can be used in some scenarios to determine behavior of
controllers.

Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>