MIPS: Add MIPS I6400 PRid and cputype identifiers

Add a Processor ID and CPU type for the MIPS I6400 core.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/10634/


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