dt-bindings: display: Add Arm virtual platforms display

'arm,rtsm-display' is a panel for Arm, Ltd. virtual platforms (e.g. FVP).
The binding has been in use for a long time, but was never documented.

Some users and an example have a 'panel-dpi' compatible, but that's not
needed without a 'panel-timing' node which none of the users have since
commit 928faf5e ("arm64: dts: fvp: Remove panel timings"). The
example does have a 'panel-timing' node, but it should not for the
same reasons the node was removed in the dts files. So update the
example in arm,pl11x.yaml to match the schema.

Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Robin Murphy <robin.murphy@arm.com>
Cc: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20220613145709.3729053-1-robh@kernel.org