common: board_r: support enable_caches for RISC-V

The enable_caches is a generic hook for architecture-implemented, we
leverage this function to enable caches for RISC-V

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

arch/riscv/lib/cache.c

@@ -70,3 +70,7 @@ __weak int dcache_status(void)
 {
 	return 0;
 }
+
+__weak void enable_caches(void)
+{
+}