dt-bindings: mhi: update devicetree doc for new transfer type support

MHI host now accept pre-mapped buffers from client for transfer,
updating devicetree documentation to reflect new support.

CRs-Fixed: 2361266
Change-Id: I4ea7cbda6ebcb6bf73d4f6f758267c5b62c6cfb4
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>