MIPS: CPC: Fix type for GCR CPC base reg for 64-bit

The GCR CPC base register is 64-bit on 64-bit processors so use the
appropriate field.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/10645/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>