MIPS: Move Alchemy Makefile parts to their own Platform file.

[Ralf: Remove a forgotten -Werror in Pb1200 Makefile.]

Signed-off-by: Manuel Lauss <manuel.lauss@googlemail.com>
To: Linux-MIPS <linux-mips@linux-mips.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Manuel Lauss <manuel.lauss@googlemail.com>
Patchwork: https://patchwork.linux-mips.org/patch/1319/


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