MIPS: BCM63XX: Cleanup cpu registers.

Use preprocessor when possible to avoid duplicated and error-prone
code.

Signed-off-by: Maxime Bizon <mbizon@freebox.fr>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2897/


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