dt-bindings: riscv: Update l2 cache DT documentation to add support for SiFive FU740

The L2 cache controller in SiFive FU740 has 4 ECC interrupt sources as
compared to 3 in FU540. Update the DT documentation accordingly with
"compatible" and "interrupt" property changes.

Signed-off-by: Yash Shah <yash.shah@sifive.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>