mhi_bus: core: add support for pre-allocating buffers for DL channels

Like other transport layers such as rpmsg, add support to
pre-allocate buffers for DL data path. So, client drivers do not
need to drastically change their implementation when switching to
a different transport layer.

CRs-Fixed: 2221013
Change-Id: I2e10089295c003a43d1b6f46c7c877a5321f5bc9
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>