ARM: dts: realview: PB11MPCore: define a standard VGA panel

Let's supply a standard VGA panel by default on the PB11MPCore,
this will work with most monitors. If more screen real estate is
desired, users can update the DPI definition.

Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Rob Herring <robh@kernel.org>
Cc: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>