clk: bindings: Fix assigned-clock-rates description

The binding uses assigned-clock-parents when it should use
assigned-clock-rates. Furthermore, the part that describes how
they relate to the assigned-clocks property is not clear about
what is related. Correct and clarify this part of the binding.

Reported-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>