firmware: scmi: use multi channel in mailbox, optee and smccc agents

Updates .process_msg operators of the SCMI transport drivers that
supports multi-channel to use it now that drivers do provide
the reference through channel argument. These are the mailbox
agent, the optee agent and the smccc agent.

Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>