MIPS: Clarify mips_cm_is64 documentation

MIPS: Clarify mips_cm_is64 documentation

The documentation for mips_cm_is64 implied that the width of the CM GCRs
would change depending upon the CPU, which is not true. Reword the
explanation to be clearer that the GCR width is purely dependent upon
the version of the CM.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Cc: James Hogan <james.hogan@imgtec.com>
Cc: Markos Chandras <markos.chandras@imgtec.com>
Patchwork: https://patchwork.linux-mips.org/patch/11185/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>