Mark switch cases where we are expecting to fall through.

Fix the following warning (Building: cavium_octeon_defconfig mips):

arch/mips/include/asm/octeon/cvmx-sli-defs.h:47:6: warning: this statement
may fall through [-Wimplicit-fallthrough=]
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>