firmware: arm_ffa: Set the core device as FF-A device parent

firmware: arm_ffa: Set the core device as FF-A device parent

Pass a parent device into ffa_device_register() and use the synthetic
arm-ffa platform device as the parent for each registered FF-A device.

This keeps the enumerated FF-A partition devices anchored below the FF-A
core device in the driver model, matching the platform-driver conversion
of the core transport.

Suggested-by: Yeoreum Yun <yeoreum.yun@arm.com>
Reviewed-by: Yeoreum Yun <yeoreum.yun@arm.com>
Link: https://patch.msgid.link/20260508-b4-ffa_plat_dev-v1-3-c5a30f8cf7b8@kernel.org
Signed-off-by: Sudeep Holla <sudeep.holla@kernel.org>