MIPS: OCTEON: cvmx-gmxx-defs.h: use default register value return when possible

If we are about to return the same register address that would
be the default anyway, fallback to default return instead of adding
a case label.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/21200/
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: linux-mips@linux-mips.org