riscv: lib: Split out support for T-Head cache management operations

riscv: lib: Split out support for T-Head cache management operations

Designed before a standard set of cache management operations defined in
RISC-V, earlier T-Head cores like C906 and C910 provide CMO through the
customized extension XTheadCMO, which has been used in the CV1800B port
of U-Boot.

This patch splits XTheadCMO-related code into a generic module, allowing
SoCs shipping T-Head cores to share the code.

Link: https://github.com/XUANTIE-RV/thead-extension-spec
Signed-off-by: Yao Zi <ziyao@disroot.org>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>