MIPS: Loongson: Cleanup the halt and poweroff action

In the old source code, I have let halt and poweroff do the same action,
but in reality, they have different meanings.

As the manpage of shutdown shows:

-r     Reboot after shutdown.
-H     Halt action is to halt or drop into boot monitor on systems that support it.
-P     Halt action is to turn off the power.

and in the real world, some machines(e.g. NAS) did not provide a power
button and the shutdown works as reset, so, we need to provide a
mechanism to let the users turn off the power safely without breaking
the system, such a mechanism is "halt", which only put the system into a
dead loop or a power-save mode and print some information to the screen
to tell the users to turn off the power safely.

$ shutdown -hH now /* loongson_halt, not turn off the power */
$ shutdown -hP now /* loongson_poweroff, work as poweroff */

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Tested-by: Liu Shiwei <liushiwei@gmail.com>
Cc: Liu Shiwei <liushiwei@gmail.com>
Cc: linux-mips <linux-mips@linux-mips.org>
Patchwork: http://patchwork.linux-mips.org/patch/883/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>