MIPS: OCTEON: Use correct instruction to read 64-bit COP0 register

Use dmfc0/dmtc0 instructions for reading CvmMemCtl COP0 register,
its a 64-bit wide.

Signed-off-by: Chandrakala Chavva <cchavva@caviumnetworks.com>
Signed-off-by: Aleksey Makarov <aleksey.makarov@auriga.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Cc: David Daney <david.daney@cavium.com>
Patchwork: https://patchwork.linux-mips.org/patch/8936/


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