MIPS: Octeon: Simplify CIU register functions.

Collapse and simplify switch statements in functions.

Signed-off-by: Steven J. Hill <steven.hill@cavium.com>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/19713/
Cc: linux-mips@linux-mips.org
Cc: Chandrakala Chavva <cchavva@caviumnetworks.com>