dt-bindings: cache: Add docs for StarFive Starlink cache controller

Add DT binding documentation used by StarFive's
Starlink cache controller.

Signed-off-by: Joshua Yeong <joshua.yeong@starfivetech.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>