drivers: scmi-msg: add SCMI MSG message protocol

Implements MSG header protocol to handle SCMI messages. MSG header
protocol was introduced in Linux kernel v5.15 [1]. It relies on normal
cached shar

drivers: scmi-msg: add SCMI MSG message protocol

Implements MSG header protocol to handle SCMI messages. MSG header
protocol was introduced in Linux kernel v5.15 [1]. It relies on normal
cached shared memory buffer using a 32bit header followed by the SCMI
message payload.

To support this message interface, the SCMI PTA defines a new capability
and a new command. Capability PTA_SCMI_CAPS_MSG_HEADER allows client and
service to negotiate the desired transport configuration. Command
PTA_SCMI_CMD_PROCESS_MSG_CHANNEL allows client to request processing
of a message sent based on that message exchange protocol.

Platforms shall enable configuration switch CFG_SCMI_MSG_SHM_MSG to have
their SCMI service supporting that communication protocol.

Link: [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f301bba0ca7392d16a6ea4f1d264a91f1fadea1a
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>
Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>

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