The current code path can lead to warnings because of uninitialized device,
which contains, as a consequence, uninitialized kobject. The uninitialized
device is passed to of_platform_populate, which will at some point, while
creating child device, try to get a reference on uninitialized parent,
resulting in the following warning:

kobject: '(null)' ((ptrval)): is not initialized, yet kobject_get() is
being called.

The warning is observed after migrating a kernel 5.10.x to 6.1.x.
Reverting commit 0d70af3c ("fpga: bridge: Use standard dev_release for
class driver") seems to remove the warning.
This commit aggregates device_initialize() and device_add() into
device_register() but this new call is done AFTER of_platform_populate

Fixes: 0d70af3c ("fpga: bridge: Use standard dev_release for class driver")
Signed-off-by: Alexis Lothoré <alexis.lothore@bootlin.com>
Acked-by: Xu Yilun <yilun.xu@intel.com>
Link: https://lore.kernel.org/r/20230404133102.2837535-2-alexis.lothore@bootlin.comSigned-off-by: Xu Yilun <yilun.xu@intel.com>