parisc: convert /proc/gsc/pcxl_dma to seq_file

As side effect, remove one more ->get_info user and a novel approach of content
generation:

	sprintf(buf, "%sfoo", buf, ...);
	sprintf(buf, "%sbar", buf, ...);
		...

Compile-tested.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>