MIPS: X1830: Add X1830 system type.

[åçæ (Zhou Yanjie)]

v1->v2:
1.Add more detailed commit message.
2.Add a /* fall-through */ comment to prevent GCC from issuing a
  warning when -Wimplicit-fallthrough is used.
3.Change my Signed-off-by from "Zhou Yanjie <zhouyanjie@xxxxxxxx>"
  to "åçæ (Zhou Yanjie) <zhouyanjie@xxxxxxxxxxxxxx>" because
  the old mailbox is in an unstable state.