drm/v3d: Add a little debugfs entry for measuring the core clock.

drm/v3d: Add a little debugfs entry for measuring the core clock.

This adds just enough performance counter support to measure the
clock.  We don't have linux kernel drivers for the clock driving the
HW, and this was useful for determining that the V3D HW is running on
a slow clock, not that the driver was slow.

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20180928232126.4332-2-eric@anholt.net
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>