Documentation: Add device tree binding for Goldfish FB driver

Add documentation for DT binding of Goldfish FB driver. The compatible
string used by OS for binding the driver is "google,goldfish-fb".

Signed-off-by: Miodrag Dinic <miodrag.dinic@mips.com>
Signed-off-by: Goran Ferenc <goran.ferenc@mips.com>
Signed-off-by: Aleksandar Markovic <aleksandar.markovic@mips.com>
Acked-by: Rob Herring <robh@kernel.org>
Cc: David Airlie <airlied@linux.ie>
Cc: Douglas Leung <douglas.leung@mips.com>
Cc: James Hogan <james.hogan@mips.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Paul Burton <paul.burton@mips.com>
Cc: Petar Jovanovic <petar.jovanovic@mips.com>
Cc: Raghu Gandham <raghu.gandham@mips.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>