MIPS: Alchemy: Simple cpu subtype detector

Extract the alchemy chip variant from c0_prid register.

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: http://patchwork.linux-mips.org/patch/14/
Patchwork: http://patchwork.linux-mips.org/patch/707/


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