irqchip/aspeed-vic: Add AST2500 compatible string

In addition to introducing the new compatible string the bindings
description is reworked to be more generic.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>