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

drivers/firmware/arm_scmi/Kconfig

@@ -30,10 +30,17 @@ config ARM_SCMI_HAVE_TRANSPORT
 	  Used to trigger a build bug when trying to build SCMI without any
 	  configured transport.
 
+config ARM_SCMI_HAVE_SHMEM
+	bool
+	help
+	  This declares whether a shared memory based transport for SCMI is
+	  available.
+
 config ARM_SCMI_TRANSPORT_MAILBOX
 	bool "SCMI transport based on Mailbox"
 	depends on MAILBOX
 	select ARM_SCMI_HAVE_TRANSPORT
+	select ARM_SCMI_HAVE_SHMEM
 	default y
 	help
 	  Enable mailbox based transport for SCMI.
@@ -45,6 +52,7 @@ config ARM_SCMI_TRANSPORT_SMC
 	bool "SCMI transport based on SMC"
 	depends on HAVE_ARM_SMCCC_DISCOVERY
 	select ARM_SCMI_HAVE_TRANSPORT
+	select ARM_SCMI_HAVE_SHMEM
 	default y
 	help
 	  Enable SMC based transport for SCMI.

drivers/firmware/arm_scmi/Makefile

 # SPDX-License-Identifier: GPL-2.0-only
 scmi-bus-y = bus.o
 scmi-driver-y = driver.o notify.o
-scmi-transport-y = shmem.o
+scmi-transport-$(CONFIG_ARM_SCMI_HAVE_SHMEM) = shmem.o
 scmi-transport-$(CONFIG_ARM_SCMI_TRANSPORT_MAILBOX) += mailbox.o
 scmi-transport-$(CONFIG_ARM_SCMI_TRANSPORT_SMC) += smc.o
 scmi-protocols-y = base.o clock.o perf.o power.o reset.o sensors.o system.o voltage.o