dt-bindings/goldfish-pic: Add device tree binding for Goldfish PIC driver

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

Acked-by: Rob Herring <robh@kernel.org>
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>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>