dt-bindings: bus: add documentation for Aspeed AHBC

Add device tree binding documentation for the Aspeed
Advanced High-Performance Bus (AHB) Controller.

Signed-off-by: Neal Liu <neal_liu@aspeedtech.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>