scmi: Add scmi_token_reserve helper

During normal operation, the SCMI stack, upon each transmission attempt,
takes care to automatically pick an appropriate, monotonically increasing,
sequence number to fill-in the token field in the egressing message.

This does not cope well with the alternative scenario in which, instead,
a complete and fully formed message is provided upfront: in such a case the
SCMI stack will have to use the already provided sequence number, embedded
in the message, to track such transaction.

Add a method to attempt to reserve a specific sequence number to be used
when a fully firmed (raw) message is handled.

Tested on: Arm Morello Board
Reviewed by: andrew
Sponsored by: Arm Ltd
Pull Request: https://reviews.freebsd.org/D47424
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>