dt-bindings: misc: aspeed-p2a-ctrl: add support

Document the ast2400, ast2500 PCI-to-AHB bridge control driver bindings.

Signed-off-by: Patrick Venture <venture@google.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>