mhi: core: Read the EXEC_ENV from device

Read the EXEC_ENV register from device to determine the MHI
base state.

Change-Id: Id70a360cfa3775a7186e5059306385a37960df2d
Signed-off-by: Andrei Danaila <adanaila@codeaurora.org>
Signed-off-by: Tony Truong <truong@codeaurora.org>