riscv: lib: implement enable_caches for sifive cache

The enable_caches is a generic hook for architecture-implemented, we
define this function to enable composable cache of sifive platforms.

In sifive_cache, it invokes the generic cache_enable interface of cache
uclass to execute the relative implementation in SiFive ccache driver.

Signed-off-by: Zong Li <zong.li@sifive.com>
Reviewed-by: Rick Chen <rick@andestech.com>