mhi: core: add support for dynamic execution environment (EE)

Some MHI based modems have different EE definitions than the
default EE value. Add devicetree support to parse device EE
definition.

CRs-Fixed: 2464900
Change-Id: I4ba14a6701d22b8aa11315f8ac245a7250b030ef
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>