firmware: arm_scmi: Assign correct parent to arm-scmi platform device

firmware: arm_scmi: Assign correct parent to arm-scmi platform device

With the decoupling of transport drivers from the SCMI core, the
"arm-scmi" platform device is now created dynamically from the probed
transport driver.

Currently, this "arm-scmi" platform device is added to the root of
the platform bus since no parent is assigned. However, the transport
platform device, created from the "firmware:scmi" node in the device
tree, should be considered its parent.

Ensure that the parent device is correctly set for the "arm-scmi"
platform device, aligning it with the correct transport hierarchy.
This will be more useful on systems with multiple transport and/or
SCMI firmware instances as the hierarchy is maintained correctly.

Message-Id: <20250317-b4-scmi_minor_cleanup-v2-4-f4be99bd9864@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>