UPSTREAM: firmware: arm_scmi: Expand SMC/HVC message pool to more than one

SMC/HVC can transmit only one message at the time as the shared memory
needs to be protected and the calls are synchronous.

However, in order to allow multiple threads to send SCMI messages
simultaneously, we need a larger poll of memory.

Let us just use value of 20 to keep it in sync mailbox transport
implementation. Any other value must work perfectly.

Link: https://lore.kernel.org/r/20201008143722.21888-4-etienne.carriere@linaro.org
Fixes: 1dc6558062 ("firmware: arm_scmi: Add smc/hvc transport")
Cc: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
[sudeep.holla: reworded the commit message to indicate the practicality]
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
(cherry picked from commit 7adb2c8aaa)

Change-Id: I36bf5833a51c7e31f822a1eac504db28519219a3
Signed-off-by: XiaoDong Huang <derrick.huang@rock-chips.com>

drivers/firmware/arm_scmi/smc.c

@@ -149,6 +149,6 @@ static struct scmi_transport_ops scmi_smc_ops = {
 const struct scmi_desc scmi_smc_desc = {
 	.ops = &scmi_smc_ops,
 	.max_rx_timeout_ms = 30,
-	.max_msg = 1,
+	.max_msg = 20,
 	.max_msg_size = 128,
 };