MIPS: ath79: drop platform device registration code

With the target now being fully OF based, we can drop the legacy platform
device registration code. All devices and their drivers are now probed
via OF.

Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Pengutronix Kernel Team <kernel@pengutronix.de>
Cc: linux-mips@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: devicetree@vger.kernel.org
Cc: Felix Fietkau <nbd@nbd.name>