MIPS: Only include the platform file needed

Instead of including all Platform files, we simply include the
needed one and avoid clashes with makefile variables.

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

MIPS: IP32: Migrate to new platform makefile style.

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