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>
diff --git a/drivers/firmware/scmi/smccc_agent.c b/drivers/firmware/scmi/smccc_agent.c
index 6a52cd7..972c6ad 100644
--- a/drivers/firmware/scmi/smccc_agent.c
+++ b/drivers/firmware/scmi/smccc_agent.c
@@ -81,6 +81,7 @@
 }
 
 static int scmi_smccc_get_channel(struct udevice *dev,
+				  struct udevice *protocol,
 				  struct scmi_channel **channel)
 {
 	struct scmi_smccc_channel *base_chan = dev_get_plat(dev);
@@ -88,7 +89,7 @@
 	u32 func_id;
 	int ret;
 
-	if (dev_read_u32(dev, "arm,smc-id", &func_id)) {
+	if (dev_read_u32(protocol, "arm,smc-id", &func_id)) {
 		/* Uses agent base channel */
 		*channel = container_of(base_chan, struct scmi_channel, ref);
 
@@ -100,7 +101,7 @@
 	if (!chan)
 		return -ENOMEM;
 
-	ret = setup_channel(dev, chan);
+	ret = setup_channel(protocol, chan);
 	if (ret) {
 		free(chan);
 		return ret;