clk: prevent erronous parsing of children during rate change

In some cases, clocks can switch their parent with clk_set_rate, for
example clk_mux can do this in some cases. Current implementation of
clk_change_rate uses un-safe list iteration on the clock children, which
will cause wrong clocks to be parsed in case any of the clock children
change their parents during the change rate operation. Fixed by using
the safe list iterator instead.

The problem was detected due to some divide by zero errors generated
by clock init on dra7-evm board, see discussion under
http://article.gmane.org/gmane.linux.ports.arm.kernel/349180

 for details.

Fixes: 71472c0c ("clk: add support for clock reparent on set_rate")
Signed-off-by: Tero Kristo <t-kristo@ti.com>
Reported-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>