clk: Avoid sending high rates to downstream clocks during set_rate

If a clock is on and we call clk_set_rate() on it we may get into
a situation where the clock temporarily increases in rate
dramatically while we walk the tree and call .set_rate() ops. For
example, consider a case where a PLL feeds into a divider.
Initially the divider is set to divide by 1 and the PLL is
running fairly slow (100MHz). The downstream consumer of the
divider output can only handle rates =< 400 MHz, but the divider
can only choose between divisors of 1 and 4.

 +-----+   +----------------+
 | PLL |-->| div 1 or div 4 |---> consumer device
 +-----+   +----------------+

To achieve a rate of 400MHz on the output of the divider, we
would have to set the rate of the PLL to 1.6 GHz and then divide
it by 4. The current code would set the PLL to 1.6GHz first while
the divider is still set to 1, thus causing the downstream
consumer of the clock to receive a few clock cycles of 1.6GHz
clock (far beyond it's maximum acceptable rate). We should be
changing the divider first before increasing the PLL rate to
avoid this problem.

Therefore, set the rate of any child clocks that are increasing
in rate from their current rate so that they can increase their
dividers if necessary. We assume that there isn't such a thing as
minimum rate requirements.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>