riscv: cache: Flush L2 cache before jump to linux

riscv: cache: Flush L2 cache before jump to linux

Flush and disable L2 cache in dcache_disable()
which will be called in cleanup_before_linux()
before jump to linux.

The sequence will be preferred as below:
L1 flush -> L1 disable -> L2 flush -> L2 disable

Signed-off-by: Rick Chen <rick@andestech.com>
Cc: KC Lin <kclin@andestech.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>