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>