clk: ti: add a usecount for autoidle

Multiple users might deny autoidle on a clock. So we should have some
counting here, also according to the comment in  _setup_iclk_autoidle().
Also setting autoidle regs is not atomic, so there is another reason
for locking.

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