MIPS: Add SysRq operation to dump TLBs on all CPUs

Add a MIPS specific SysRq operation to dump the TLB entries on all CPUs,
using the 'x' trigger key.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/10072/


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