firmware: arm_scmi: Add transport instance handles

firmware: arm_scmi: Add transport instance handles

SCMI transport drivers are initialized first and then the control is passed
to the SCMI core stack: some of these transports are dependent also on some
external subsytem which will have to be initialized upfront, before the
transport driver itself can be deemed operational.

Transport drivers like virtio or optee need a way to defer the core SCMI
probing till they are fully initialized and operational and also a way to
pass back the device reference to be used as a supplier while building the
devlink relations.

SCMI transport drivers can be probed multiple times when used in a multiple
instance configuration but the capability to carry-on with multiple probes
depends on the support provided by the underlying transport driver.

This change will also allow for the removal of the frowned-upon trick of
registering a platform driver only after the end of the transport drivers
porbe to avoid explicit probe deferrals.

Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Link: https://patch.msgid.link/20260510160527.3537474-2-cristian.marussi@arm.com
Signed-off-by: Sudeep Holla <sudeep.holla@kernel.org>