MIPS: Allow read64 GCR accessors to work on MIPS32 kernels

MIPS: Allow read64 GCR accessors to work on MIPS32 kernels

If we run a MIPS32 kernel on a system using CM3 we may still need to
access 64 bit GCRs, as will be done in later patches. Allow this by
having the read64_gcr_* accessor functions perform 2 x 32 bit reads on
those systems.

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/11188/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>