scmi: Add helper to manipulate scmi_msg descriptors

[cristian.marussi_arm.com]

Summary

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
Sponsored by: Arm Ltd
Signed-off-by: Cristian Marussi <[e-mail]>