firmware: arm_scmi: Add atomic mode support to smc transport

Add a Kernel configuration option to enable SCMI SMC transport atomic
mode operation for selected SCMI transactions and leave it as default
disabled.

Substitute mutex usages with busy-waiting and declare smc transport as
.atomic_enabled if such Kernel configuration option is enabled.

Link: https://lore.kernel.org/r/20211220195646.44498-8-cristian.marussi@arm.com


Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>