media: sun6i-mipi-csi2: Register async subdev with no sensor attached

This allows the device to probe and register its async subdev without
a sensor attached.

The rationale is that the parent driver might otherwise wait for the
subdev to be registered when it should be available (from the fwnode
graph endpoint perspective). This is generally not problematic when
the MIPI CSI-2 bridge is the only device attached to the parent, but
in the case of a CSI controller that can feed from both MIPI CSI-2
and parallel, it would prevent using the parallel sensor due to the
parent waiting for the MIPI CSI-2 subdev to register.

Fixes: af54b4f4 ("media: sunxi: Add support for the A31 MIPI CSI-2 controller")
Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>