riscv: Add SiFive CCache driver.

Eswin EIC7700 has non-coherent DMAs but predate the standard RISC-V Zicbom
extension, so we need to use the SiFive CCache controller for non-standard
cache management operations.

Tested on SiFive Premier P550.

Reviewed by: mhorne, jrtc27
Differential Revision: https://reviews.freebsd.org/D47831