drm/pl111: Add a debugfs node to dump our registers.

While debugging an X11 display failure, I wanted to see where we were
actually scanning out from.  This is probably generally useful to
others that might be working on this device.

v2: Fix uint32_t sparse warning.

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: http://patchwork.freedesktop.org/patch/msgid/20170518005640.10310-1-eric@anholt.net
Reviewed-by: Linus Walleij <linus.walleij@linaro.org> (v1)