firmware: arm_scmi: Make shmem support optional for transports

Upcoming new SCMI transports won't need any kind of shared memory support.
Compile shmem.c only if a shmem based transport is selected.

Link: https://lore.kernel.org/r/20210803131024.40280-10-cristian.marussi@arm.com


Co-developed-by: Peter Hilber <peter.hilber@opensynergy.com>
Signed-off-by: Igor Skalkin <igor.skalkin@opensynergy.com>
[ Peter: Adapted patch for submission to upstream. ]
Signed-off-by: Peter Hilber <peter.hilber@opensynergy.com>
[ Cristian: Adapted patch/commit_msg to new SCMI Kconfig layout ]
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>