mhi: core: add support for 64bit doorbell address

Doorbell pointers are 64bit addresses. Use correct
data type when passing the address.

CRs-Fixed: 2038601
Change-Id: Ie8e42d38944c0529386b90f5a6d37dcd77c19184
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>