MIPS: Allow kernel to use coprocessor 2

Kernel threads should be able to use COP2 if the platform needs it.
Do not call die_if_kernel() for a coprocessor unusable exception if
the exception due to COP2 usage.  Instead, the default notifier for
COP2 exceptions is updated to call die_if_kernel.

Signed-off-by: Jayachandran C <jchandra@broadcom.com>
Cc: linux-mips@linux-mips.org
Cc: ddaney.cavm@gmail.com
Patchwork: https://patchwork.linux-mips.org/patch/5415/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>