mhi: core: Expose API to check device execution environment

Expose an API for clients to check the device execution
environment by reading the BHI_EXECENV register. This can be
used in situations where the controller decides to check the
state of a device which can abruptly reset without warning.

Change-Id: Ib0841535f8baf7bbdf8e737a816a400ee072ab93
Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>