firmware: add SCMI agent uclass

firmware: add SCMI agent uclass

This change introduces SCMI agent uclass to interact with a firmware
using the SCMI protocols [1].

SCMI agent uclass currently supports a single method to request
processing of the SCMI message by an identified server. A SCMI message
is made of a byte payload associated to a protocol ID and a message ID,
all defined by the SCMI specification [1]. On return from process_msg()
method, the caller gets the service response.

SCMI agent uclass defines a post bind generic sequence for all devices.
The sequence binds all the SCMI protocols listed in the FDT for that
SCMI agent device. Currently none, but later change will introduce
protocols.

This change implements a simple sandbox device for the SCMI agent uclass.
The sandbox nicely answers SCMI_NOT_SUPPORTED to SCMI messages.
To prepare for further test support, the sandbox exposes a architecture
function for test application to read the sandbox emulated devices state.
Currently supports 2 SCMI agents, identified by an ID in the FDT device
name. The simplistic DM test does nothing yet.

SCMI agent uclass is designed for platforms that embed a SCMI server in
a firmware hosted somewhere, for example in a companion co-processor or
in the secure world of the executing processor. SCMI protocols allow an
SCMI agent to discover and access external resources as clock, reset
controllers and more. SCMI agent and server communicate following the
SCMI specification [1]. This SCMI agent implementation complies with
the DT bindings defined in the Linux kernel source tree regarding
SCMI agent description since v5.8.

Links: [1] https://developer.arm.com/architectures/system-architectures/software-standards/scmi
Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Cc: Simon Glass <sjg@chromium.org>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Sudeep Holla <sudeep.holla@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>