dmaengine: pl330: introduce debugfs interface

This patch adds debugfs interface to show the relationship between
DMA threads (hardware resource for transferring data) and DMA
channel ID of DMA slave.

Typically, PL330 has many slaves than number of DMA threads.
So sometimes PL330 cannot allocate DMA threads for all slaves even
if a user specify DMA channel ID in devicetree. This interface will
be useful for checking that DMA threads are allocated or not.

Below is an output sample:

$ sudo cat /sys/kernel/debug/ff1f0000.dmac
PL330 physical channels:
THREAD:         CHANNEL:
--------        -----
0               8
1               9
2               11
3               12
4               14
5               15
6               10
7               --

Signed-off-by: Katsuhiro Suzuki <katsuhiro@katsuster.net>
Signed-off-by: Vinod Koul <vkoul@kernel.org>