drm/pl111: Support Integrator IM-PD1 module

The last in-kernel user of the old framebuffer driver is the
IM-PD1 module for the Integrator/AP. Let's implement support for
this remaining user so we can migrate the last user over to
DRM and delete the old FB driver.

On the Integrator/AP the IM-PD1 system controller will exist
alongside the common Integrator system controller so make
sure to do a special lookup for the IM-PD1 syscon and make it
take precedence if found.

Tested on the Integrator/AP with the IM-PD1 mounted.

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200213124833.35545-1-linus.walleij@linaro.org