mhi: core: Create dummy MHI APIs to fix compile errors

MHI is not supported by all targets, however many MHI client drivers
have a common driver that include MHI APIs. Create dummy APIs such
that even if MHI not present, does not cause any compile errors.

CRs-Fixed: 2053159
Change-Id: I427d925a658518d230a1d14df0cf27a7b19f207e
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>