MIPS: Loongson: Cleanup machtype support

To choose code for different machines by the value of machtype it needs to
be initialized as early as possible.  So move initialization of
mips_machtype to prom_init().

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