drm/panel: Add simple panel mode for the ARM RTSM

drm/panel: Add simple panel mode for the ARM RTSM

Having failed any attempts at a more generic solution,
I fall back to the very specific solution: define a simple
panel for the ARM RTSM emulated platforms.

I am doing this so we can convert all old users from the
previous fbdev driver to the PL111 DRM driver.

This works fine as far as I can test, provided the
device tree for RTSM AEMv8 is augmented accordingly.

Cc: Sudeep Holla <sudeep.holla@arm.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Mali DP Maintainers <malidp@foss.arm.com>
Cc: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20181026111334.3365-1-linus.walleij@linaro.org