drm/pl111: Enable PL110 variant

We detect and enable the use of the PL110 variant, an earlier
incarnation of PL111. The only real difference is that the
control and interrupt enable registers have swapped place.
The Versatile AB and Versatile PB have a variant inbetween
PL110 and PL111, it is PL110 but they have already swapped the
two registers so those two need a bit of special handling.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20170908124709.4758-4-linus.walleij@linaro.org