scmi: Add helper to manipulate scmi_msg descriptors

Refactor allocation logic for scmi_req and introduce new helpers to be able
to obtain an scmi_msg reference to a freshly allocated request.

Tested on: Arm Morello Board
Reviewed by: andrew
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D47423
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>