RISC-V: sifive_l2_cache: Add L2 cache controller driver for SiFive SoCs

The driver currently supports only SiFive FU540-C000 platform.

The initial version of L2 cache controller driver includes:
- Initial configuration reporting at boot up.
- Support for ECC related functionality.

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