staging/vchi: Add a TODO file of things I know we need to deal with.

I've left out the downstream HDMI audio driver from the "to be
imported" section, as we'll want to handle it natively in vc4.  The
downstream kernel will likely continue to use that driver for a while
due to using the interim "vc4 firmware KMS" mode while vc4 gets its
featureset (such as HDMI audio!) completed.

I've also left out VC-CMA, which appears to be about having Linux
manage a CMA area that the firmware gets to make allocations out of.
I'm not clear on if this is useful (the firmware's need for memory
drops massively with vc4 present, and may drop even more depending on
how we resolve dmabuf handling for camera and video decode)

Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>