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: 1dc65580 ("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>