riscv: dts: Add DT support for SiFive L2 cache controller

Add the L2 cache controller DT node in SiFive FU540 soc-specific DT file

Signed-off-by: Yash Shah <yash.shah@sifive.com>
Reviewed-by: Palmer Dabbelt <palmerdabbelt@google.com>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>