clk: fix set_rate to clean up cached rates for the hierarchy

Clock rates are cached within the individual clock nodes, and right now
if one changes a clock rate somewhere in the middle of the tree, none
of its child clocks notice the change. To fix this, clear up all the
cached rates for us and our child clocks.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Tero Kristo <kristo@kernel.org>