clk: ti: move clk_hw_omap list handling under generic part of the driver

Currently the clk_hw_omap list is handled under the autoidle code, but
it should be accessible generically. Add a few APIs towards this, and
update the autoidle code to use the generic implementations.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Tested-by: Keerthy <j-keerthy@ti.com>