dmaengine: nbpfaxi: Provide some missing attribute docs and split out slave info

Fixes the following W=1 kernel build warning(s):

 drivers/dma/nbpfaxi.c:157: warning: Function parameter or member 'chan' not described in 'nbpf_desc'
 drivers/dma/nbpfaxi.c:220: warning: Function parameter or member 'tasklet' not described in 'nbpf_channel'
 drivers/dma/nbpfaxi.c:220: warning: Function parameter or member 'slave_src_addr' not described in 'nbpf_channel'
 drivers/dma/nbpfaxi.c:220: warning: Function parameter or member 'slave_src_width' not described in 'nbpf_channel'
 drivers/dma/nbpfaxi.c:220: warning: Function parameter or member 'slave_src_burst' not described in 'nbpf_channel'
 drivers/dma/nbpfaxi.c:220: warning: Function parameter or member 'slave_dst_addr' not described in 'nbpf_channel'
 drivers/dma/nbpfaxi.c:220: warning: Function parameter or member 'slave_dst_width' not described in 'nbpf_channel'
 drivers/dma/nbpfaxi.c:220: warning: Function parameter or member 'slave_dst_burst' not described in 'nbpf_channel'
 drivers/dma/nbpfaxi.c:220: warning: Function parameter or member 'running' not described in 'nbpf_channel'
 drivers/dma/nbpfaxi.c:220: warning: Function parameter or member 'paused' not described in 'nbpf_channel'

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: Song Liu <songliubraving@fb.com>
Cc: Yonghong Song <yhs@fb.com>
Cc: Andrii Nakryiko <andriin@fb.com>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: KP Singh <kpsingh@chromium.org>
Cc: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Cc: netdev@vger.kernel.org
Cc: bpf@vger.kernel.org
Link: https://lore.kernel.org/r/20200714111546.1755231-14-lee.jones@linaro.org


Signed-off-by: Vinod Koul <vkoul@kernel.org>