[ARM] S3C: BUG_ON() if clock has already been registered

[ARM] S3C: BUG_ON() if clock has already been registered

Add a simple check when registering a clock on whether
the clock has already been added to the list.

Any attempt to re-register a clock will cause the
clock list to be come looped and thus produces silent
failures when looking up clocks.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>