dbdma: add per-channel debugging enabled via DEBUG_DBDMA_CHANMASK

By default large amounts of DBDMA debugging are produced when often it is just
1 or 2 channels that are of interest. Introduce DEBUG_DBDMA_CHANMASK to allow
the developer to select the channels of interest at compile time, and then
further add the extra channel information to each debug statement where
possible.

Also clearly mark the start/end of DBDMA_run_bh to allow tracking the bottom
half execution.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>