ARM: OMAP3: clockdomain: fix accidental duplicate registration

Commit 472fd5401561f94698f4c8f9dbbbfbf76ab55626 (Merge branch 'cleanup-hwmod'
into cleanup) inadvertently caused a clockdomain to be registered twice on
OMAP3 boards.  This causes warnings on boot, wild pointer dereferences, and
PM regressions.  Fix the double registration and add some clockdomain code
to prevent this from happening again.

Reported-by: Joe Woodward <jw@terrafix.co.uk>
Acked-by: Kevin Hilman <khilman@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>
[tony@atomide.com: updated commit message for the breaking merge]
Signed-off-by: Tony Lindgren <tony@atomide.com>