MIPS: Add R16000 detection

This allows the kernel to correctly detect an R16000 MIPS CPU on systems that
have those.  Otherwise, such systems will detect the CPU as an R14000, due to
similarities in the CPU PRId value.

Signed-off-by: Joshua Kinard <kumba@gentoo.org>
Cc: Linux MIPS List <linux-mips@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/9092/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>