bus: ti-sysc: Fix iterating over clocks

Commit d878970f ("bus: ti-sysc: Add separate functions for handling
clocks") separated handling of optional clocks from the main clocks, but
introduced an issue where we do not necessarily allocate a slot for both
fck and ick clocks, but still assume fixed slots for enumerating over the
clocks.

Let's fix the issue by ensuring we always have slots for both fck and ick
even if we don't use ick, and don't attempt to enumerate optional clocks
if not allocated.

In the long run we might want to simplify things a bit by only allocating
space only for the optional clocks as we have only few devices with
optional clocks.

Fixes: d878970f ("bus: ti-sysc: Add separate functions for handling clocks")
Signed-off-by: Tony Lindgren <tony@atomide.com>