firmware: scmi: add configs to select the supported agents

Add two configs CONFIG_SCMI_AGENT_MAILBOX and CONFIG_SCMI_AGENT_SMCCC
to select the supported agents as all the agents are not supported.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Acked-by: Etienne Carriere <etienne.carriere@linaro.org>
diff --git a/drivers/firmware/scmi/Makefile b/drivers/firmware/scmi/Makefile
index 966475e..7d6f4df 100644
--- a/drivers/firmware/scmi/Makefile
+++ b/drivers/firmware/scmi/Makefile
@@ -1,5 +1,5 @@
 obj-y	+= scmi_agent-uclass.o
 obj-y	+= smt.o
-obj-$(CONFIG_ARM_SMCCC)		+= smccc_agent.o
-obj-$(CONFIG_DM_MAILBOX)	+= mailbox_agent.o
+obj-$(CONFIG_SCMI_AGENT_SMCCC)		+= smccc_agent.o
+obj-$(CONFIG_SCMI_AGENT_MAILBOX)	+= mailbox_agent.o
 obj-$(CONFIG_SANDBOX)		+= sandbox-scmi_agent.o sandbox-scmi_devices.o