Command to be sent via the stream interface are written to a local
memory page, whose address is then provided to the GSC.
The interface supports providing a full sg with multiple pages for both
input and output messages, but since for now we only aim to support short
and synchronous messages we can use a single page for both input and
output.

Note that the mei interface expects an sg of 4k pages, while our lmem pages
are 64k. If we ever need to support more than 4k we'll need to convert.
Added a TODO comment to the code to record this.
Signed-off-by: Vitaly Lubart <vitaly.lubart@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Alan Previn <alan.previn.teres.alexis@intel.com>
Reviewed-by: Alan Previn <alan.previn.teres.alexis@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220928004145.745803-9-daniele.ceraolospurio@intel.com