bus: ti-sysc: Add fck clock alias for children with notifier_block

The functional clock is used by several child device drivers to query
the rate for the child device internal configuration. The functional
clock is really for the whole interconnect target module, and not just
for the child device, and can also be shared across multiple children.
At least the timers, i2c and mmc driver query the fck for rate.

So let's just create a clock alias for the child fck if it does not
yet exits. We can do this with the BUS_NOTIFY_ADD_DEVICE before the
child is probed.

Note that we need to now also remove the legacy mode check for getting
the dts clocks in ti-sysc driver.

Signed-off-by: Tony Lindgren <tony@atomide.com>