MIPS: X1830: Add X1830 system type.

From: åçæ (Zhou Yanjie)
Date: Mon Dec 09 2019 - 00:10:59 EST

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.