drm/arcpgu: Opt in debugfs

This change adopts debugfs usage for outputting useful data.
As of today we print:
 * Mode and real HW clock values
 * Standard FB info

Code is heavily borrowed from ARM's HDLCD thus adding Liviu in Cc.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Reviewed-by: Liviu Dudau <Liviu.Dudau@arm.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: David Airlie <airlied@linux.ie>
Cc: Jose Abreu <joabreu@synopsys.com>