firmware: arm_scpi: reinit completion instead of full init_completion()

Instead of performing full initialization of the completion structure
on each transfer in scpi_send_message(), we initialize it at boot time
(more specifically, in the relevant probe() function) and use
reinit_completion() to reset ->done counter on each message transfer.

Signed-off-by: Alexey Klimov <alexey.klimov@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>