firmware: scmi: use a protocol's own channel if assigned

firmware: scmi: use a protocol's own channel if assigned

SCMI specification allows any protocol to have its own channel for
the transport. While the current SCMI driver may assign its channel
from a device tree, the core function, devm_scmi_process_msg(), doesn't
use a protocol's channel, but always use an agent's channel.

With this commit, devm_scmi_process_msg() tries to find and use
a protocol's channel. If it doesn't exist, use an agent's.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Etienne Carriere <etienne.carriere@foss.st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>