media: staging/imx: remove static subdev arrays

For more complex OF graphs, there will be more async subdevices
registered. Remove the static subdev[IMX_MEDIA_MAX_SUBDEVS] array,
so that imx-media places no limits on the number of async subdevs
that can be added and registered.

There were two uses for 'struct imx_media_subdev'. First was to act
as the async subdev list to be passed to v4l2_async_notifier_register().

Second was to aid in inheriting subdev controls to the capture devices,
and this is done by creating a list of capture devices that can be reached
from a subdev's source pad. So 'struct imx_media_subdev' also contained
a static array of 'struct imx_media_pad' for placing the capture device
lists at each pad.

'struct imx_media_subdev' has been completely removed. Instead, at async
completion, allocate an array of 'struct imx_media_pad' and attach it to
the subdev's host_priv pointer, in order to support subdev controls
inheritance.

Likewise, remove static async_ptrs[IMX_MEDIA_MAX_SUBDEVS] array.
Instead, allocate a 'struct imx_media_async_subdev' when forming
the async list, and add it to an asd_list list_head in
imx_media_add_async_subdev(). At async completion, allocate the
asd pointer list and pull the asd's off asd_list for
v4l2_async_notifier_register().

Signed-off-by: Steve Longerbeam <steve_longerbeam@mentor.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>