clk: si5351: Set initial clkout rate when defined in platform data.

clock-frequency property from platform data was read but never used.
Apply defined rate when clock is registered.

Signed-off-by: Marek Belisko <marek.belisko@streamunlimited.com>
Acked-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>
[mturquette@linaro.org: add missing changelog]
Cc: stable@kernel.org

Signed-off-by: Mike Turquette <mturquette@linaro.org>