Documentation/sysrq.txt: Mention MIPS TLB dump (x)

Documentation/sysrq.txt: Mention MIPS TLB dump (x)

Commit d1e9a4f54735 ("MIPS: Add SysRq operation to dump TLBs on all
CPUs") added the 'x' sysrq key for dumping MIPS TLB entries, but didn't
document it in Documentation/sysrq.txt.

Add mention of the MIPS use of the 'x' SysRq key.

Reported-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Acked-by: Jonathan Corbet <corbet@lwn.net>
Cc: Maciej W. Rozycki <macro@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: linux-doc@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/10720/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>