This patch revises `k230_hart.[ch]` by:
- revising big core boot/stop control.
- making k230_hart_is_big() available in S-mode.
- adding more comments.
This patch also revises the `ld-kernel.script` so that to match the
latest MMU pgtable design.
Signed-off-by: Yanfeng Liu <yfliu2008@qq.com>