fpga: dfl: factor out feature device registration

fpga: dfl: factor out feature device registration

Add separate functions, feature_dev_{register,unregister}(), that wrap
platform_device_add() and platform_device_unregister(), respectively.
These are invoked once per feature device in this commit but will be
reused in a subsequent commit to destroy and recreate the platform
device when the corresponding port is released and reassigned.

The function feature_dev_register() will be extended in subsequent
commits to allocate the platform device, add resources and platform
data, and finally add the platform device to the device hierarchy.

Signed-off-by: Peter Colberg <peter.colberg@intel.com>
Reviewed-by: Matthew Gerlach <matthew.gerlach@linux.intel.com>
Reviewed-by: Basheer Ahmed Muddebihal <basheer.ahmed.muddebihal@linux.intel.com>
Acked-by: Xu Yilun <yilun.xu@intel.com>
Link: https://lore.kernel.org/r/20241120011035.230574-10-peter.colberg@intel.com
Signed-off-by: Xu Yilun <yilun.xu@linux.intel.com>