drm/pl111: Add all registers to debugfs

This adds all the main control registers to the debugfs
register file. This was helpful for my debugging so it will
likely help others as well.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20170908124709.4758-2-linus.walleij@linaro.org