Rename siba -> siba_s5, to specifically reference that it's for the
legacy siba sentry5 cpu glue.

The siba_cc code is the hard-coded chipcommon bits for the sentry s5,
which will eventually be replaced with the more flexible bhnd sipa/cc
code.

bwn, etc uses siba_bwn, which doesn't use siba or siba_cc to do anything.