OMAP clockdomain/powerdomain: remove runtime register/unregister

OMAP clockdomains and powerdomains are currently defined statically,
only registered at boot, and never unregistered, so we can remove the
unregister function and the locking.   A variant of this was originally
suggested a while ago by Dmitry Baryshkov <dbaryshkov@gmail.com>.
This version of this patch contains an additional fix from Kevin Hilman
<khilman@deeprootsystems.com> to address one of the pwrdm_for_each_nolock()
users in mach-omap2/pm-debug.c.   Thanks Kevin.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Dmitry Baryshkov <dbaryshkov@gmail.com>
Cc: Kevin Hilman <khilman@deeprootsystems.com>