bus: ti-sysc: Make child clock alias handling more generic

In order to prepare supporting clkctrl optional clocks, we need to
make the current child clock handling more generic so we can use the
clock role names for the optional clocks in the following patch.

Cc: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>