nubus: Switch to dynamic root device

nubus: Switch to dynamic root device

Driver core expects devices to be dynamically allocated and will, for
example, complain loudly if a device that lacks a release function is
ever freed.

Use root_device_register() to allocate and register the root device
instead of open coding using a static device.

Signed-off-by: Johan Hovold <johan@kernel.org>
Acked-by: Finn Thain <fthain@linux-m68k.org>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://patch.msgid.link/20260424104011.2616970-1-johan@kernel.org
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>