MIPS: Move Cobalt Makefile parts to their own Platform file

Signed-off-by: Yoichi Yuasa <yuasa@linux-mips.org>
Cc: linux-mips <linux-mips@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/1321/


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