scmi: Add optional asynchronous handling of replies

Add the core logic to handle the replies to pending command in an
asynchronous manner.

Tested on: Arm Morello Board
Reviewed by: andrew
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D47426
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>