RISC-V: Add DT documentation for SiFive L2 Cache Controller

Add device tree bindings for SiFive FU540 L2 cache controller driver

Signed-off-by: Yash Shah <yash.shah@sifive.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>