clk: ti: interface: add support for legacy interface clock init

Legacy clock data is initialized slightly differently compared to
DT clocks, thus add support for this. The interface clock descriptor
itself is overloading the gate clock descriptor, thus it needs to
be called from the gate setup.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Michael Turquette <mturquette@linaro.org>