dt-bindings: Document Loongson RS780E PCH ACPI Controller

This controller is attached under ISA Bus and can be found
in Loongson-3 systems with RS780E PCH.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>