MIPS: Alchemy: use runtime cpu detection in GPIO code.

Remove the cpu subtype cpp macros in favor of runtime detection,
to improve compile coverage of the alchemy common code.
(Increases kernel size by 700 bytes).

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: http://patchwork.linux-mips.org/patch/699/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>