MIPS: Loongson: Add a machtype kernel command line argument

The difference between some loongson-based machines is very small, so, if
there is no necessary to add new kernel config options to cope with this
difference, it will be better to share the same kernel image file between
them, benefit from this, the linux distribution developers only have a need
to compile the kernel one time.

This machtype kernel command line argument will be used later to share the
same kernel image file between two different machines(menglong & yeeloong)
made by lemote.

Thanks very much to Zhang Le for cleaning up the machtype implementation.

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>