MIPS: Don't panic on 5KEc.

It's a bloody bog standard MIPS64R2 core with just a new PrId ID.  Iow
that essentially means Linux just panics because it doesn't know how to
name the core.

[ralf@linux-mips.org: Split original patch into several smaller patches.]

Signed-off-by: Leonid Yegoshin <yegoshin@mips.com>
Signed-off-by: Steven J. Hill <sjhill@mips.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3792/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>