firmware: introduce SCMI core support

Introduce core support for ARM's SCMI (System Control and
Management Interface). This includes:

* shared memory (SHMEM) driver. This consists of a suite
of functions used to interact with the shared memory area.

* shared memory and doorbell-based transport layer driver.
Data is passed between platform and agent via shared
memory. Signaling is done using polling (PRE_KERNEL) and
doorbells (POST_KERNEL). This makes use of Zephyr MBOX API
(for signaling purposes) and the SHMEM driver (for polling
and data transfer).

* core driver - acts as glue between transport and protocol
layers. Provides synchronized access to transport layer
channels and channel assignment/initialization.

* infrastructure for creating SCMI protocols

This is based on ARM's SCMI Platform Design Document: DEN0056E.

Signed-off-by: Laurentiu Mihalcea <laurentiu.mihalcea@nxp.com>