firmware: arm_scmi: Provide a missing function param description

gcc as well as clang now produce warnings for missing kerneldoc function
parameter.

Fix the following W=1 kernel build warning:

drivers/firmware/arm_scmi/smc.c:32:
 warning: Function parameter or member 'shmem_lock' not described in 'scmi_smc'

Link: https://lore.kernel.org/r/20200709153155.22573-1-sudeep.holla@arm.com


Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>