ARM: dts: Augment VGA connector bridge on Realview PBX

The PL111 in the ARM reference platforms are connected to
"panels" that are actually dumb VGA DAC connector bridges.
Now that we can support the proper bridges in the DRM driver,
fix this up.

Cc: Mali DP Maintainers <malidp@foss.arm.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>