RISC-V: sifive_l2_cache: Update L2 cache driver to support SiFive FU740

SiFive FU740 has 4 ECC interrupt sources as compared to 3 in FU540.
Update the L2 cache controller driver to support this additional
interrupt in case of FU740-C000 chip.

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